Re: [RFC PATCH v3] ppc/spapr: Add support for H_SCM_PERFORMANCE_STATS hcall

[Vaibhav Jain]

Thanks for looking into this patch David and Groug,

David Gibson <david@gibson.dropbear.id.au> writes:
> On Sat, May 15, 2021 at 01:07:59PM +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 encoded as an unsigned
>> integer (e.g 'MemLife ' == 0x4D656D4C69666520) and its value be a
>> 8-byte unsigned 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>
>> ---
>> Changelog
>> 
>> Since RFC-v2:
>> * s/SCM_STATS_MAX_OUTPUT_BUFFER/SCM_STATS_MIN_OUTPUT_BUFFER/ thats the
>>   minimum size buffer needed to return all supported performance
>>   stats. Value for this is derived from size of array 'nvdimm_perf_stats'
>> * Added SCM_STATS_MAX_OUTPUT_BUFFER to indicate maximum supported
>>   rr-buffer size from a guest. Value for this is derived from hard
>>   limit of SCM_STATS_MAX_STATS.
>> * Updated scm_perf_check_rr_buffer() to add a check for max size of
>>   rr-buffer. [David]
>> * Updated scm_perf_check_rr_buffer() to removed a reduntant check for
>>   min size of rr-buffer [David]
>> * Updated h_scm_performance_stats() to have a single allocation for
>>   rr-buffer and copy it back to guest memory in a single shot. [David]
>> 
>> Since RFC-v1:
>> * Removed empty lines from code [ David ]
>> * Updated struct papr_scm_perf_stat to use uint64_t for
>>   statistic_id.
>> * Added a hard limit on max number of stats requested to 255 [ David ]
>> * Updated scm_perf_check_rr_buffer() to check for rr-buffer header
>>   size [ David ]
>> * Removed a redundant check from nvdimm_stat_getval() [ David ]
>> * Removed a redundant call to address_space_access_valid() in
>>   scm_perf_check_rr_buffer() [ David ]
>> * Instead of allocating a minimum size local buffer, allocate a max
>>   possible size local rr-buffer. [ David ]
>> * Updated nvdimm_stat_getval() to set 'val' to '0' on error. [ David ]
>> * Updated h_scm_performance_stats() to use a canned-response method
>>   for simplifying num_stats==0 case [ David ].
>> ---
>>  hw/ppc/spapr_nvdimm.c  | 222 +++++++++++++++++++++++++++++++++++++++++
>>  include/hw/ppc/spapr.h |  19 +++-
>>  2 files changed, 240 insertions(+), 1 deletion(-)
>> 
>> diff --git a/hw/ppc/spapr_nvdimm.c b/hw/ppc/spapr_nvdimm.c
>> index 252204e25f..287326b9c0 100644
>> --- a/hw/ppc/spapr_nvdimm.c
>> +++ b/hw/ppc/spapr_nvdimm.c
>> @@ -35,6 +35,19 @@
>>  /* SCM device is unable to persist memory contents */
>>  #define PAPR_PMEM_UNARMED PPC_BIT(0)
>>  
>> +/* Minimum output buffer size needed to return all nvdimm_perf_stats */
>> +#define SCM_STATS_MIN_OUTPUT_BUFFER  (sizeof(struct papr_scm_perf_stats) + \
>> +                                      sizeof(struct papr_scm_perf_stat) * \
>> +                                      ARRAY_SIZE(nvdimm_perf_stats))
>
> MIN_OUTPUT_BUFFER is a better name, but still not great.  I think we
> can get rid of this define completely in a neat way, though.  See below.
>
>
Not sure how we can get rid of it since we still need to advertise to
the guest how much rr-buffer size we expect to return all
perf-stats. Sorry but I didnt make out of  any suggestions below that
could get rid of this define.


>> +/* Maximum number of stats that we can return back in a single stat request 
>> */
>> +#define SCM_STATS_MAX_STATS 255
>
> Although it's technically allowed, I'm still not convinced there's
> actually any reason to allow the user to request the same stat over
> and over.
>
Matching the PowerVM behaviour here which doesnt enforce any limitations
on the how many times a single perf-stat can appear in rr-buffer.

>> +/* Maximum possible output buffer to fulfill a perf-stats request */
>> +#define SCM_STATS_MAX_OUTPUT_BUFFER  (sizeof(struct papr_scm_perf_stats) + \
>> +                                      sizeof(struct papr_scm_perf_stat) * \
>> +                                      SCM_STATS_MAX_STATS)
>> +
>>  bool spapr_nvdimm_validate(HotplugHandler *hotplug_dev, NVDIMMDevice 
>> *nvdimm,
>>                             uint64_t size, Error **errp)
>>  {
>> @@ -502,6 +515,214 @@ static target_ulong h_scm_health(PowerPCCPU *cpu, 
>> SpaprMachineState *spapr,
>>      return H_SUCCESS;
>>  }
>>  
>> +static int perf_stat_noop(SpaprDrc *drc, uint64_t unused, uint64_t *val)
>> +{
>> +    *val = 0;
>> +    return H_SUCCESS;
>> +}
>> +
>> +static int perf_stat_memlife(SpaprDrc *drc, uint64_t unused, uint64_t *val)
>> +{
>> +    /* Assume full life available of an NVDIMM right now */
>> +    *val = 100;
>> +    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];
>
> So using a char[] here, but a uint64_t in the request structure makes
> it pretty hard to follow if you're doing the right thing
> w.r.t. endianness, especially since you effectively memcmp() directly
> between u64s and char[]s.  You really want to use a consistent type
> for the ids.
>
Though the PAPR-SCM defines stat-ids as u64 they are essentially 8-byte
ascii strings encoded in a u64. The guest kernel and this proposed qemu
patch doesnt do any math operations on them which might be effected by
their endianess.

The switch from u64->char[8] is done only for a more convinent
ASCII representation stats-ids in nvdimm_pref_stats[].

>> +    int  (*stat_getval)(SpaprDrc *drc, uint64_t id, 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, uint64_t id, uint64_t *val)
>> +{
>> +    int index;
>> +    char stat_id[8];
>> +
>> +    /* since comparision is done */
>> +    memcpy(&stat_id[0], &id, 8);
>
> I don't see why you're making this temporary copy here.
>
Agree, removed this in next iteration of this patch.
>> +    *val = 0;
>> +
>> +    /* Lookup the stats-id in the nvdimm_perf_stats table */
>> +    for (index = 0; index < ARRAY_SIZE(nvdimm_perf_stats); ++index) {
>> +        if (memcmp(nvdimm_perf_stats[index].stat_id, &stat_id[0], 8) == 0) {
>> +            return nvdimm_perf_stats[index].stat_getval(drc, id, val);
>> +        }
>> +    }
>> +    return H_PARTIAL;
>> +}
>> +
>> +/*
>> + * Given a request & result buffer header verify its contents. Also
>> + * buffer-size and number of stats requested are within our expected
>> + * sane bounds.
>> + */
>> +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;
>> +
>> +    /* 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_MIN_OUTPUT_BUFFER;
>> +    } else if (*num_stats > SCM_STATS_MAX_STATS) {
>> +        /* Too many stats requested */
>> +        return H_P3;
>
> I'd recommend testing and exiting on this error case before handling
> the all stats case.  Disposing of error cases early is more idiomatic.
>
> You can then combine the all stats and n-stats cases a bit more nicely
> with something like:
>     actual_numstats = (*num_stats) ? (*num_stats) : ARRAY_SIZE(...);
>
> Then use the same logic to compute the expected bufsize (min_bufsize
> might be a better name) in both cases.
>       
>
Agree, have done the change you suggested in next iteration.

>> +    } else { /* Return a subset of stats */
>> +        expected_buffsize = sizeof(struct papr_scm_perf_stats) +
>> +            (*num_stats) * sizeof(struct papr_scm_perf_stat);
>> +    }
>> +
>> +    if (size < expected_buffsize || size > SCM_STATS_MAX_OUTPUT_BUFFER) {
>> +        return H_P3;
>
> I think you can avoid the MAX_OUTPUT_BUFFER check here...
>
Yes, moved the check to h_scm_performance_stats() in next iteration.

>> +    }
>> +
>> +    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)
>> +{
>> +    SpaprDrc *drc = spapr_drc_by_index(args[0]);
>> +    const hwaddr addr = args[1];
>> +    size_t size = args[2];
>> +    struct papr_scm_perf_stats *perfstats;
>> +    struct papr_scm_perf_stat *stats;
>> +    uint64_t invalid_stat = 0, stat_val;
>> +    MemTxResult result;
>> +    uint32_t num_stats;
>> +    unsigned long rc;
>> +    int 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;
>> +    }
>> +
>> +    /* verify size is enough to hold rr-buffer header */
>> +    if (size < sizeof(struct papr_scm_perf_stats)) {
>
>
> .. if you put it here instead, then you will have dealt with all
> obviously bad buffer sizes early.
>
Agree
>> +        return H_BAD_DATA;
>> +    }
>> +
>> +    /* allocate enough buffer space locally for holding max stats */
>> +    perfstats = g_try_malloc0(SCM_STATS_MAX_OUTPUT_BUFFER);
>
> Then you can safely base this malloc on the given size, rather than
> always over-allocating.
>
Right, have updated this in v4
>> +    if (!perfstats) {
>> +        return H_NO_MEM;
>> +    }
>> +
>> +    /* Read and verify rr-buffer header */
>> +    result = address_space_read(&address_space_memory, addr,
>> +                                MEMTXATTRS_UNSPECIFIED, perfstats,
>> +                                sizeof(struct papr_scm_perf_stats));
>
> And you can also read the entire thing with a single memory read here.
>
Yes agree. Addressed this in v4.
>> +    rc = (result == MEMTX_OK) ?
>> +        scm_perf_check_rr_buffer(perfstats, addr, size, &num_stats) :
>> +        H_PRIVILEGE;
>
> This is a bit cryptic.  Just deal with the memtx error first, then run
> the buffer validation.  Actually, you can unify the exit paths for
> these and the success case by using a goto label near the end which
> has the g_free() and return rc.
>
Sure, addressed this in v4 by using g_autofree
>> +    if (rc != H_SUCCESS) {
>> +        g_free(perfstats);
>> +        return rc;
>> +    }
>> +
>> +    stats = &perfstats->scm_statistics[0];
>> +    /* when returning all stats generate a canned response first */
>> +    if (num_stats == 0) {
>> +        for (index = 1; index < ARRAY_SIZE(nvdimm_perf_stats); ++index) {
>> +            memcpy(&stats[index - 1].statistic_id,
>> +                   &nvdimm_perf_stats[index].stat_id, 8);
>> +        }
>> +        num_stats = ARRAY_SIZE(nvdimm_perf_stats) - 1;
>> +    } else {
>> +        /* 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(perfstats);
>> +            return H_PRIVILEGE;
>> +        }
>> +    }
>> +
>> +    for (index = 0; index < num_stats; ++index) {
>> +        rc = nvdimm_stat_getval(drc, stats[index].statistic_id, &stat_val);
>> +
>> +        /* On error add noop stat to rr buffer & save last inval stat-id */
>> +        if (rc != H_SUCCESS) {
>> +            if (!invalid_stat) {
>> +                invalid_stat = stats[index].statistic_id;
>> +            }
>> +            memcpy(&stats[index].statistic_id, 
>> nvdimm_perf_stats[0].stat_id, 8);
>> +        }
>> +        /* Caller expects stat values in BE encoding */
>> +        stats[index].statistic_value = cpu_to_be64(stat_val);
>> +    }
>> +
>> +    /* Update and copy the local rr-buffer back to guest */
>> +    perfstats->num_statistics = cpu_to_be32(num_stats);
>> +    g_assert(size <= SCM_STATS_MAX_OUTPUT_BUFFER);
>> +    result = address_space_write(&address_space_memory, addr,
>> +                                 MEMTXATTRS_UNSPECIFIED, perfstats, size);
>> +
>> +    /* Cleanup the stats buffer */
>> +    g_free(perfstats);
>> +    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 +732,7 @@ 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..6f3353b4ea 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 {
>> +    uint64_t statistic_id;
>> +    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

-- 
Cheers
~ Vaibhav