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Re: [RFC-PATCH] ppc/spapr: Add support for H_SCM_PERFORMANCE_STATS hcall
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From: |
Vaibhav Jain |
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Subject: |
Re: [RFC-PATCH] ppc/spapr: Add support for H_SCM_PERFORMANCE_STATS hcall |
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Date: |
Thu, 06 May 2021 08:49:09 +0530 |
Hi David,
Firstly, apologies for responding so late.
I have spinned off a v2 of this RFC patch addressing your recent review
comments at
20210506024924.85526-1-vaibhav@linux.ibm.com">https://lore.kernel.org/qemu-devel/20210506024924.85526-1-vaibhav@linux.ibm.com
--
Cheers
~ Vaibhav
David Gibson <david@gibson.dropbear.id.au> writes:
> On Tue, Apr 20, 2021 at 03:06:18PM +0530, Vaibhav Jain wrote:
>> Thanks for looking into this patch David,
>>
>> David Gibson <david@gibson.dropbear.id.au> writes:
>>
>> > On Thu, Apr 15, 2021 at 01:23:43PM +0530, Vaibhav Jain wrote:
>> >> Add support for H_SCM_PERFORMANCE_STATS described at [1] for
>> >> spapr nvdimms. This enables guest to fetch performance stats[2] like
>> >> expected life of an nvdimm ('MemLife ') etc and display them to the
>> >> user. Linux kernel support for fetching these performance stats and
>> >> exposing them to the user-space was done via [3].
>> >>
>> >> The hcall semantics mandate that each nvdimm performance stats is
>> >> uniquely identied by a 8-byte ascii string (e.g 'MemLife ') and its
>> >> value be a 8-byte integer. These performance-stats are exchanged with
>> >> the guest in via a guest allocated buffer called
>> >> 'requestAndResultBuffer' or rr-buffer for short. This buffer contains
>> >> a header descibed by 'struct papr_scm_perf_stats' followed by an array
>> >> of performance-stats described by 'struct papr_scm_perf_stat'. The
>> >> hypervisor is expected to validate the rr-buffer header and then based
>> >> on the request copy the needed performance-stats to the array of
>> >> 'struct papr_scm_perf_stat' following the header.
>> >>
>> >> The patch proposes a new function h_scm_performance_stats() that
>> >> services the H_SCM_PERFORMANCE_STATS hcall. After verifying the
>> >> validity of the rr-buffer header via scm_perf_check_rr_buffer() it
>> >> proceeds to fill the rr-buffer with requested performance-stats. The
>> >> value of individual stats is retrived from individual accessor
>> >> function for the stat which are indexed in the array
>> >> 'nvdimm_perf_stats'.
>> >>
>> >> References:
>> >> [1] "Hypercall Op-codes (hcalls)"
>> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/powerpc/papr_hcalls.rst#n269
>> >> [2] Sysfs attribute documentation for papr_scm
>> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/sysfs-bus-papr-pmem#n36
>> >> [3] "powerpc/papr_scm: Fetch nvdimm performance stats from PHYP"
>> >> https://lore.kernel.org/r/20200731064153.182203-2-vaibhav@linux.ibm.com
>> >>
>> >> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
>> >> ---
>> >> hw/ppc/spapr_nvdimm.c | 243 +++++++++++++++++++++++++++++++++++++++++
>> >> include/hw/ppc/spapr.h | 19 +++-
>> >> 2 files changed, 261 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/hw/ppc/spapr_nvdimm.c b/hw/ppc/spapr_nvdimm.c
>> >> index 252204e25f..4830eae4a4 100644
>> >> --- a/hw/ppc/spapr_nvdimm.c
>> >> +++ b/hw/ppc/spapr_nvdimm.c
>> >> @@ -35,6 +35,11 @@
>> >> /* SCM device is unable to persist memory contents */
>> >> #define PAPR_PMEM_UNARMED PPC_BIT(0)
>> >>
>> >> +/* Maximum output buffer size needed to return all nvdimm_perf_stats */
>> >> +#define SCM_STATS_MAX_OUTPUT_BUFFER (sizeof(struct papr_scm_perf_stats)
>> >> + \
>> >> + sizeof(struct papr_scm_perf_stat)
>> >> * \
>> >> + ARRAY_SIZE(nvdimm_perf_stats))
>> >> +
>> >> bool spapr_nvdimm_validate(HotplugHandler *hotplug_dev, NVDIMMDevice
>> >> *nvdimm,
>> >> uint64_t size, Error **errp)
>> >> {
>> >> @@ -502,6 +507,243 @@ static target_ulong h_scm_health(PowerPCCPU *cpu,
>> >> SpaprMachineState *spapr,
>> >> return H_SUCCESS;
>> >> }
>> >>
>> >> +static int perf_stat_noop(SpaprDrc *drc, uint8_t unused[8], uint64_t
>> >> *val)
>> >> +{
>> >> + *val = 0;
>> >> + return H_SUCCESS;
>> >> +}
>> >> +
>> >> +static int perf_stat_memlife(SpaprDrc *drc, uint8_t unused[8], uint64_t
>> >> *val)
>> >> +{
>> >> + /* Assume full life available of an NVDIMM right now */
>> >> + *val = 100;
>> >
>> > AFAICT the reporting mechanism makes basically all the stats
>> > optional. Doesn't it make more sense to omit stats, rather than use
>> > dummy values in this case? Or is this just an example for the RFC?
>> >
>> Yes, this was just an RFC example to illustrate adding support for a new
>> stat.
>
> Ok.
>
>> >> + return H_SUCCESS;
>> >> +}
>> >> +
>> >> +/*
>> >> + * Holds all supported performance stats accessors. Each
>> >> performance-statistic
>> >> + * is uniquely identified by a 8-byte ascii string for example: 'MemLife
>> >> '
>> >> + * which indicate in percentage how much usage life of an nvdimm is
>> >> remaining.
>> >> + * 'NoopStat' which is primarily used to test support for retriving
>> >> performance
>> >> + * stats and also to replace unknown stats present in the rr-buffer.
>> >> + *
>> >> + */
>> >> +static const struct {
>> >> + char stat_id[8];
>> >> + int (*stat_getval)(SpaprDrc *drc, uint8_t id[8], uint64_t *val);
>> >> +} nvdimm_perf_stats[] = {
>> >> + { "NoopStat", perf_stat_noop},
>> >> + { "MemLife ", perf_stat_memlife},
>> >> +};
>> >> +
>> >> +/*
>> >> + * Given a nvdimm drc and stat-name return its value. In case given
>> >> stat-name
>> >> + * isnt supported then return H_PARTIAL.
>> >> + */
>> >> +static int nvdimm_stat_getval(SpaprDrc *drc, uint8_t id[8], uint64_t
>> >> *val)
>> >> +{
>> >> + int index;
>> >> +
>> >> + /* Lookup the stats-id in the nvdimm_perf_stats table */
>> >> + for (index = 0; index < ARRAY_SIZE(nvdimm_perf_stats); ++index) {
>> >> +
>> >
>> > No blank line here.
>> >
>> Sure, will fix the blank line from this and other places you reported.
>> >> + if (memcmp(nvdimm_perf_stats[index].stat_id, &id[0], 8) == 0 &&
>> >> + nvdimm_perf_stats[index].stat_getval) {
>> >
>> > I don't see any reason you'd want an entry in the table with a NULL
>> > function, so I don't think you need both tests.
>> >
>> Right. Was being extra cautious here.
>> >> +
>> >
>> > No blank line here either.
>> >
>> >> + return nvdimm_perf_stats[index].stat_getval(drc, id, val);
>> >> + }
>> >> + }
>> >> +
>> >> + return H_PARTIAL;
>> >> +}
>> >> +
>> >> +/*
>> >> + * Given a request & result buffer header verify its contents. Also
>> >> + * verify that buffer & buffer-size provided by the guest is accessible
>> >> and
>> >> + * large enough to hold the requested stats. The size of buffer needed to
>> >> + * hold the requested 'num_stat' number of stats is returned in 'size'.
>> >> + */
>> >> +static int scm_perf_check_rr_buffer(struct papr_scm_perf_stats *header,
>> >> + hwaddr addr, size_t *size,
>> >> + uint32_t *num_stats)
>> >> +{
>> >> + size_t expected_buffsize;
>> >> +
>> >
>> > You need to check that size is at least big enough to contain the
>> > header before accessing the header fields.
>> >
>> Yes, the expected_buffsize variable already calculated and checks for
>> the space needed for for header + space for stats.
>
> I don't follow what you're saying. You read both the eyecatcher and
> version from the structure before you ever set expected_buffsize.
>
>> >> + /* Verify the header eyecather and version */
>> >> + if (memcmp(&header->eye_catcher, SCM_STATS_EYECATCHER,
>> >> + sizeof(header->eye_catcher))) {
>> >> + return H_BAD_DATA;
>> >> + }
>> >> + if (be32_to_cpu(header->stats_version) != 0x1) {
>> >> + return H_NOT_AVAILABLE;
>> >> + }
>> >> +
>> >> + /* verify that rr buffer has enough space */
>> >> + *num_stats = be32_to_cpu(header->num_statistics);
>> >> + if (*num_stats == 0) { /* Return all stats */
>> >> + expected_buffsize = SCM_STATS_MAX_OUTPUT_BUFFER;
>> >> + } else { /* Return a subset of stats */
>> >> + expected_buffsize = sizeof(struct papr_scm_perf_stats) +
>> >> + (*num_stats) * sizeof(struct papr_scm_perf_stat);
>> >> +
>> >> + }
>> >
>> > We probably want a hard cap on num_stats as well, so the guest can't
>> > force up to make arbitrarily large allocations and memory read/writes.
>> >
>> Agree. Though the papr spec doesnt provide any upper bound on number of
>> stats that can be requested, I think a hard cap can be 255 which is the
>> max number of stats that a 4K page can hold.
>
> So, that's one possibility. But, although it's technically permitted
> for a guest to provide a big buffer asking for the same stat over and
> over, is there actually any reason to support such a pointless way of
> using the interface? If not, could we just set the hard cap to the
> number of different stats we support.
>
>>
>> >> +
>> >> + if (*size < expected_buffsize) {
>> >> + return H_P3;
>> >> + }
>> >> + *size = expected_buffsize;
>> >> +
>> >> + /* verify that rr buffer is writable */
>> >> + if (!address_space_access_valid(&address_space_memory, addr, *size,
>> >> + true, MEMTXATTRS_UNSPECIFIED)) {
>> >
>> > Is there any point to this, given that you'll still have to check for
>> > errors when you go to write back the buffer later?
>> >
>> Yes, agree. Will get rid of this check in next iteration.
>>
>> >> + return H_PRIVILEGE;
>> >> + }
>> >> +
>> >> + return H_SUCCESS;
>> >> +}
>> >> +
>> >> +/*
>> >> + * For a given DRC index (R3) return one ore more performance stats of
>> >> an nvdimm
>> >> + * device in guest allocated Request-and-result buffer (rr-buffer) (R4)
>> >> of
>> >> + * given 'size' (R5). The rr-buffer consists of a header described by
>> >> + * 'struct papr_scm_perf_stats' that embeds the 'stats_version' and
>> >> + * 'num_statistics' fields. This is followed by an array of
>> >> + * 'struct papr_scm_perf_stat'. Based on the request type the writes the
>> >> + * performance into the array of 'struct papr_scm_perf_stat' embedded
>> >> inside
>> >> + * the rr-buffer provided by the guest.
>> >> + * Special cases handled are:
>> >> + * 'size' == 0 : Return the maximum possible size of rr-buffer
>> >> + * 'size' != 0 && 'num_statistics == 0' : Return all possible
>> >> performance stats
>> >> + *
>> >> + * In case there was an error fetching a specific stats (e.g stat-id
>> >> unknown or
>> >> + * any other error) then return the stat-id in R4 and also replace its
>> >> stat
>> >> + * entry in rr-buffer with 'NoopStat'
>> >> + */
>> >> +static target_ulong h_scm_performance_stats(PowerPCCPU *cpu,
>> >> + SpaprMachineState *spapr,
>> >> + target_ulong opcode,
>> >> + target_ulong *args)
>> >> +{
>> >> + const uint32_t drc_index = args[0];
>> >> + const hwaddr addr = args[1];
>> >> + size_t size = args[2];
>> >> + int index;
>> >> + MemTxResult result;
>> >> + uint32_t num_stats;
>> >> + uint8_t stat_id[8];
>> >> + unsigned long rc;
>> >> + uint64_t stat_val, invalid_stat = 0;
>> >> + struct papr_scm_perf_stats perfstats;
>> >> + struct papr_scm_perf_stat *stats, *stat;
>> >> + SpaprDrc *drc = spapr_drc_by_index(drc_index);
>> >> +
>> >> + /* Ensure that the drc is valid & is valid PMEM dimm and is plugged
>> >> in */
>> >> + if (!drc || !drc->dev ||
>> >> + spapr_drc_type(drc) != SPAPR_DR_CONNECTOR_TYPE_PMEM) {
>> >> + return H_PARAMETER;
>> >> + }
>> >> +
>> >> + /* Guest requested max buffer size for output buffer */
>> >> + if (size == 0) {
>> >> + args[0] = SCM_STATS_MAX_OUTPUT_BUFFER;
>> >> + return H_SUCCESS;
>> >> + }
>> >> +
>> >> + /* Read and verify rr-buffer header */
>> >> + result = address_space_read(&address_space_memory, addr,
>> >> + MEMTXATTRS_UNSPECIFIED, &perfstats,
>> >> + sizeof(perfstats));
>> >
>> > Ah.. actually you need to check that the provided size is at least big
>> > enough to cover the header before even reading it here.
>> >
>> But that verification is already being done in
>> scm_perf_check_rr_buffer() as part of header check. In case the provided
>> buffer is less than sizeof(perfstats) than address_space_read() would
>> fail or susequently scm_perf_check_rr_buffer() would return an
>> error.
>
> So, yes, you'd probably get away with it. I really dislike relying on
> "wrong but we'll catch it later" logic. I really prefer to perform
> the bounds check *before* performing the guest memory access.
>
>> >> + if (result != MEMTX_OK) {
>> >> + return H_PRIVILEGE;
>> >> + }
>> >> +
>> >> + /* Verify that the rr-buffer is valid */
>> >> + rc = scm_perf_check_rr_buffer(&perfstats, addr, &size, &num_stats);
>> >> + if (rc != H_SUCCESS) {
>> >> + return rc;
>> >> + }
>> >> +
>> >> + /* allocate enough buffer space locally for holding all stats */
>> >> + stats = g_malloc0(size - sizeof(struct papr_scm_perf_stats));
>> >
>> > This seems unnecessarily complicated. Why not just allocate a max
>> > sized temporary buffer every time - it's in the tens of bytes, not
>> > something that is really a concern from a memory usage point of view.
>> > You could even put it on the stack.
>> >
>>
>> 'SCM_STATS_MAX_OUTPUT_BUFFER' is the minimum size of output buffer
>> needed to hold all the supported nvdimm stats. A guest can always send a
>> buffer sized larger than 'SCM_STATS_MAX_OUTPUT_BUFFER' where single stat
>> is requested more than once. In such a case 'bufferSizeInBytes'
>> (args[2]) received from guest which is a uint64 can be large.
>
> Ah.. yes, I forgot about that option. Mind you if you set the max
> number of requests to the max number of stats as suggested above, you
> could simplify this path as well.
>
>> review comment to self: add check for memory allocation failure
>
> You don't need that; g_malloc() will abort() on allocation failure.
>
>>
>> >> + if (num_stats == 0) { /* Return all supported stats */
>> >> +
>> >
>> > No blank line here.
>> >
>> >> + for (index = 1; index < ARRAY_SIZE(nvdimm_perf_stats); ++index) {
>> >
>> > Why is the starting index 1, not 0?
>> >
>> Dont want to return noopstat to the guest. 'nvdimm_perf_stat' will
>> always start with a noopstat descriptor hence want to skip that.
>>
>> >> + stat = &stats[index - 1];
>> >> + memcpy(stat_id, &nvdimm_perf_stats[index].stat_id, 8);
>> >
>> > I don't see any point to the 'stat_id' variable here.
>> >
>> Right, thanks for pointin it out. This lingered from an earlier version
>> of the patch and I will get it removed.
>>
>> >> + rc = nvdimm_stat_getval(drc, stat_id, &stat_val);
>> >
>> > So, you're using the nvdimm_stat_getval() here in the num_stats==0
>> > path, which means you're not taking advatage of the fact that you
>> > don't actually need to search through the table for your getter
>> > function in this case. I think that's reasonable for its simplicity,
>> > but in that case you can make it even simpler:
>> >
>> > Rather than having separate paths for the num_stats == 0 and the other
>> > case, just have the num_stats == 0 case fill in the buffer with a
>> > canned request which asks for each stat in turn. Then continue on to
>> > the selected stats path.
>> >
>> Thanks, will implement something similar in next iteration.
>>
>>
>> >> +
>> >> + /* On error add noop stat to rr buffer & save last inval
>> >> stat-id */
>> >> + if (rc != H_SUCCESS) {
>> >> + if (!invalid_stat) {
>> >> + memcpy(&invalid_stat, &stat_id[0], 8);
>> >> + }
>> >> + memcpy(&stat_id, nvdimm_perf_stats[0].stat_id, 8);
>> >> + stat_val = 0;
>> >> + }
>> >> +
>> >> + memcpy(&stat->statistic_id, stat_id, 8);
>> >> + stat->statistic_value = cpu_to_be64(stat_val);
>> >> + }
>> >> + /* Number of stats returned == perf_stats array size - noop-stat
>> >> */
>> >> + num_stats = ARRAY_SIZE(nvdimm_perf_stats) - 1;
>> >> +
>> >> + } else { /* Return a subset of requested stats */
>> >> +
>> >
>> > No blank line.
>> >
>> >> + /* copy the rr-buffer from the guest memory */
>> >> + result = address_space_read(&address_space_memory,
>> >> + addr + sizeof(struct
>> >> papr_scm_perf_stats),
>> >> + MEMTXATTRS_UNSPECIFIED, stats,
>> >> + size - sizeof(struct
>> >> papr_scm_perf_stats));
>> >> +
>> >> + if (result != MEMTX_OK) {
>> >> + g_free(stats);
>> >> + return H_PRIVILEGE;
>> >> + }
>> >> +
>> >> + for (index = 0; index < num_stats; ++index) {
>> >> + stat = &stats[index];
>> >> + memcpy(&stat_id, &stats->statistic_id, 8);
>> >
>> > What's the point of the 'stat_id' temporary?
>> >
>> Agree as mentioned earlier. Will remove this in next iteration of this
>> patch.
>>
>> >> + rc = nvdimm_stat_getval(drc, stat_id, &stat_val);
>> >> +
>> >> + /* On error add noop stat to rr buffer & save last inval
>> >> stat-id */
>> >> + if (rc != H_SUCCESS) {
>> >> + if (!invalid_stat) {
>> >> + memcpy(&invalid_stat, &stat_id[0], 8);
>> >> + }
>> >> + memcpy(&stat_id, nvdimm_perf_stats[0].stat_id, 8);
>> >
>> > Why not write back directly to (the qemu copy of) the rr buffer?
>> >
>> Sure can do that. Was trying to update qemu rr-buffer at a single
>> point rather than scattering updates to it at multiple places.
>>
>>
>> >> + stat_val = 0;
>> >
>> >
>> > You can also avoid the explicit stat_val = 0 if you make
>> > nvdimm_stat_getval() always zero stat_val on error.
>> >
>> Agree. Thanks for the suggestion. Will add this in next iteration.
>>
>> >> + }
>> >> +
>> >> + memcpy(&stat->statistic_id, stat_id, 8);
>> >
>> > AFAICT this copy only does something in the failure case.
>> >
>> Right
>>
>> >> + stat->statistic_value = cpu_to_be64(stat_val);
>> >> + }
>> >> + }
>> >> +
>> >> + /* Update and copy the local rr-buffer header and stats back to
>> >> guest */
>> >> + perfstats.num_statistics = cpu_to_be32(num_stats);
>> >> + result = address_space_write(&address_space_memory, addr,
>> >> + MEMTXATTRS_UNSPECIFIED, &perfstats,
>> >> + sizeof(struct papr_scm_perf_stats));
>> >> + if (result == MEMTX_OK) {
>> >> + result = address_space_write(&address_space_memory,
>> >> + addr + sizeof(struct
>> >> papr_scm_perf_stats),
>> >> + MEMTXATTRS_UNSPECIFIED, stats,
>> >> + size - sizeof(struct
>> >> papr_scm_perf_stats));
>> >> + }
>> >> +
>> >> + /* Cleanup the stats buffer */
>> >> + g_free(stats);
>> >> +
>> >> + if (result) {
>> >> + return H_PRIVILEGE;
>> >> + }
>> >> +
>> >> + /* Check if there was a failure in fetching any stat */
>> >> + args[0] = invalid_stat;
>> >> + return invalid_stat ? H_PARTIAL : H_SUCCESS;
>> >> +}
>> >> +
>> >> static void spapr_scm_register_types(void)
>> >> {
>> >> /* qemu/scm specific hcalls */
>> >> @@ -511,6 +753,7 @@ static void spapr_scm_register_types(void)
>> >> spapr_register_hypercall(H_SCM_UNBIND_MEM, h_scm_unbind_mem);
>> >> spapr_register_hypercall(H_SCM_UNBIND_ALL, h_scm_unbind_all);
>> >> spapr_register_hypercall(H_SCM_HEALTH, h_scm_health);
>> >> + spapr_register_hypercall(H_SCM_PERFORMANCE_STATS,
>> >> h_scm_performance_stats);
>> >> }
>> >>
>> >> type_init(spapr_scm_register_types)
>> >> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
>> >> index d2b5a9bdf9..4b71b58e00 100644
>> >> --- a/include/hw/ppc/spapr.h
>> >> +++ b/include/hw/ppc/spapr.h
>> >> @@ -326,6 +326,7 @@ struct SpaprMachineState {
>> >> #define H_P8 -61
>> >> #define H_P9 -62
>> >> #define H_OVERLAP -68
>> >> +#define H_BAD_DATA -70
>> >> #define H_UNSUPPORTED_FLAG -256
>> >> #define H_MULTI_THREADS_ACTIVE -9005
>> >>
>> >> @@ -539,8 +540,9 @@ struct SpaprMachineState {
>> >> #define H_SCM_UNBIND_MEM 0x3F0
>> >> #define H_SCM_UNBIND_ALL 0x3FC
>> >> #define H_SCM_HEALTH 0x400
>> >> +#define H_SCM_PERFORMANCE_STATS 0x418
>> >>
>> >> -#define MAX_HCALL_OPCODE H_SCM_HEALTH
>> >> +#define MAX_HCALL_OPCODE H_SCM_PERFORMANCE_STATS
>> >>
>> >> /* The hcalls above are standardized in PAPR and implemented by pHyp
>> >> * as well.
>> >> @@ -787,6 +789,21 @@ OBJECT_DECLARE_SIMPLE_TYPE(SpaprTceTable,
>> >> SPAPR_TCE_TABLE)
>> >> DECLARE_INSTANCE_CHECKER(IOMMUMemoryRegion, SPAPR_IOMMU_MEMORY_REGION,
>> >> TYPE_SPAPR_IOMMU_MEMORY_REGION)
>> >>
>> >> +/* Defs and structs exchanged with guest when reporting drc perf stats */
>> >> +#define SCM_STATS_EYECATCHER "SCMSTATS"
>> >> +
>> >> +struct QEMU_PACKED papr_scm_perf_stat {
>> >> + uint8_t statistic_id[8];
>> >> + uint64_t statistic_value;
>> >> +};
>> >> +
>> >> +struct QEMU_PACKED papr_scm_perf_stats {
>> >> + uint8_t eye_catcher[8]; /* Should be “SCMSTATS” */
>> >> + uint32_t stats_version; /* Should be 0x01 */
>> >> + uint32_t num_statistics; /* Number of stats following */
>> >> + struct papr_scm_perf_stat scm_statistics[]; /* Performance matrics */
>> >> +};
>> >> +
>> >> struct SpaprTceTable {
>> >> DeviceState parent;
>> >> uint32_t liobn;
>> >
>>
>
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_
> _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
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- Re: [RFC-PATCH] ppc/spapr: Add support for H_SCM_PERFORMANCE_STATS hcall,
Vaibhav Jain <=