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Re: [Qemu-ppc] [PATCH 1/1] spapr: Check SMT based on KVM_CAP_PPC_SMT_POS
From: |
Greg Kurz |
Subject: |
Re: [Qemu-ppc] [PATCH 1/1] spapr: Check SMT based on KVM_CAP_PPC_SMT_POSSIBLE |
Date: |
Thu, 11 Jan 2018 16:00:42 +0100 |
On Wed, 10 Jan 2018 22:14:37 -0200
address@hidden wrote:
> On Tue, Jan 09, 2018 at 01:48:13PM +0100, Greg Kurz wrote:
> > On Fri, 5 Jan 2018 22:47:22 -0200
> > Jose Ricardo Ziviani <address@hidden> wrote:
> >
> > > Power9 supports 4 HW threads/core but it's possible to emulate
> > > doorbells to implement virtual SMT. KVM has the KVM_CAP_PPC_SMT_POSSIBLE
> > > which returns a bitmap with all SMT modes supported by the host.
> > >
> > > Today, QEMU forces the SMT mode based on PVR compat table, this is
> > > silently done in spapr_fixup_cpu_dt. Then, if user passes thread=8 the
> > > guest will end up with 4 threads/core without any feedback to the user.
> > > It is confusing and will crash QEMU if a cpu is hotplugged in that
> > > guest.
> > >
> > > This patch makes use of KVM_CAP_PPC_SMT_POSSIBLE to check if the host
> > > supports the SMT mode so it allows Power9 guests to have 8 threads/core
> > > if desired.
> > >
> > > Reported-by: Satheesh Rajendran <address@hidden>
> > > Signed-off-by: Jose Ricardo Ziviani <address@hidden>
> > > ---
> >
> > Hi,
> >
> > I agree with the general idea but I have a few questions.
>
> Hello!!!! Thank you for your detailed review. :)
>
> I'm copying David too because I've seen other bugs related with (v)smt
> topic (specially migration) that it could address.
>
David was already in the Cc list, as expected for all ppc patches :)
> >
> > The MIN(smp_threads, ppc_compat_max_threads(cpu)) computation is
> > performed in spapr_fixup_cpu_dt() at CAS, but it is also performed
> > in spapr_populate_cpu_dt() at machine reset or when a CPU is added.
> >
> > Shouldn't your patch address the latter as well ?
>
> As far as I investigated, I found out that ppc_compat_max_threads() is
> called several times, but it always returns the number of threads from
> the argument line. Only in spapr_fixup_cpu_dt(), that happens during
> the guest kernel initialization when it's realizing the CPUS, is that
I guess you mean 'onlining the CPUS' ?
> ppc_compat_max_threads() will return that MIN(n_threads, compat->max_threads).
>
> Until them, if(cpu->compat_pvr) is zeroed and QEMU doesn't know the
> max_threads yet.
>
Indeed, compat mode is negotiated during CAS which happens before secondary
cpus onlining... but ppc_compat_max_threads() is also called when we hot-plug
cpus, at a time QEMU should know about compat->max_threads.
Speaking of that, it looks like we have another bug in this case: hot-plugged
cpus are exposed with a real PVR in the DT...
address@hidden ~]# dtc -f -I fs -O dts /proc/device-tree | grep cpu-version
cpu-version = <0x4e1200>;
cpu-version = <0xf000005>;
cpu-version = <0xf000005>;
Should ppc_set_compat() should be called before we populate the FDT nodes of
hot-plugged cpus, instead of of calling it from rtas_start_cpu() ?
> That's the reason that I added the code only in spapr_fixup_cpu_dt()
> because this is where the change really happens.
>
> >
> > > hw/ppc/spapr.c | 14 +++++++++++++-
> > > hw/ppc/trace-events | 1 +
> > > target/ppc/kvm.c | 5 +++++
> > > target/ppc/kvm_ppc.h | 6 ++++++
> > > 4 files changed, 25 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index d1acfe8858..ea2503cd2f 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -345,7 +345,19 @@ static int spapr_fixup_cpu_dt(void *fdt,
> > > sPAPRMachineState *spapr)
> > > PowerPCCPU *cpu = POWERPC_CPU(cs);
> > > DeviceClass *dc = DEVICE_GET_CLASS(cs);
> > > int index = spapr_vcpu_id(cpu);
> > > - int compat_smt = MIN(smp_threads, ppc_compat_max_threads(cpu));
> >
> > Considering that we have:
> >
> > int ppc_compat_max_threads(PowerPCCPU *cpu)
> > {
> > const CompatInfo *compat = compat_by_pvr(cpu->compat_pvr);
> > int n_threads = CPU(cpu)->nr_threads;
> >
> > if (cpu->compat_pvr) {
> > g_assert(compat);
> > n_threads = MIN(n_threads, compat->max_threads);
> > }
> >
> > return n_threads;
> > }
> >
> > and
> >
> > void qemu_init_vcpu(CPUState *cpu)
> > {
> > cpu->nr_cores = smp_cores;
> > cpu->nr_threads = smp_threads;
> > ...
> > }
> >
> > ppc_compat_max_threads() already returns the smaller value of
> > smp_threads and the maximum number of HW threads for the PVR.
> >
> > I don't quite understand why we had this compat_smt calculation
> > in the first place...
>
> Mostly it only returns "n_threads = CPU(cpu)->nr_threads" because until
> the guest kernel initialization cpu->compat_pvr is false so that
> MIN() macro is not executed.
>
I'm referring to the computation in spapr_fixup_cpu_dt(), which basically
does:
compat_smt = MIN(smp_threads, MIN(smp_threads, compat->max_threads));
> So, until late, QEMU thinks its guest will have 8 threads/core. During
> the guest kernel init., that fixup code calls ppc_compat_max_threads
> that will now have cpu->compat_pvr true and will change the number
> of threads to 4. Example:
>
> qemu-system-ppc64 -smp sockets=1,cores=1,threads=8
> +-> qemu_init_vcpu, spapr_populate_cpu_dt: 8 threads/core
> +-> guest kernel is running and asks about CPUs, spapr_fixup_cpu_dt()
> runs, sets threads to 4, set ibm,ppc-interrupt-server#s and done.
> +-> guest now believes it only has 4 threads.
>
> >
> > > +
> > > + /* set smt to maximum for this current pvr if the number
> > > + * passed is higher than defined by PVR compat mode AND
> > > + * if KVM cannot emulate it.*/
> > > + int compat_smt = smp_threads;
> > > + if ((kvmppc_cap_smt_possible() & smp_threads) != smp_threads &&
> > > + smp_threads > ppc_compat_max_threads(cpu)) {
> > > + compat_smt = ppc_compat_max_threads(cpu);
> > > +
> > > + trace_spapr_fixup_cpu_smt(index, smp_threads,
> > > + kvmppc_cap_smt_possible(),
> > > + ppc_compat_max_threads(cpu));
> > > + }
> >
> > ... so I'm wondering if the above shouldn't be performed in
> > ppc_compat_max_threads() directly ?
>
> Hmm, now I'm believe that the whole code could rely on that
> kvmppc_cap_smt_possible() since it will always return the number of
> threads supported by the underlying HW. We could have a check in the
Not always, it may return 0 with older KVM or TCG...
> very beginning:
>
> if ((kvmppc_cap_smt_possible() & smp_threads) != smp_threads) {
... in which case, the check ^^ would be wrong.
> // explain the user that such setup is wrong and quit.
> }
>
> and that part in fixup code could be unecessary.
>
> >
> > >
> > > if ((index % smt) != 0) {
> > > continue;
> > > diff --git a/hw/ppc/trace-events b/hw/ppc/trace-events
> > > index b7c3e64b5e..a8e29d7ab1 100644
> > > --- a/hw/ppc/trace-events
> > > +++ b/hw/ppc/trace-events
> > > @@ -16,6 +16,7 @@ spapr_irq_alloc(int irq) "irq %d"
> > > spapr_irq_alloc_block(int first, int num, bool lsi, int align) "first
> > > irq %d, %d irqs, lsi=%d, alignnum %d"
> > > spapr_irq_free(int src, int irq, int num) "Source#%d, first irq %d, %d
> > > irqs"
> > > spapr_irq_free_warn(int src, int irq) "Source#%d, irq %d is already free"
> > > +spapr_fixup_cpu_smt(int idx, int smpt, int kvmt, int pvrt) "CPU(%d):
> > > expected smt %d, kvm support %d, max smt pvr %d"
> > >
> > > # hw/ppc/spapr_hcall.c
> > > spapr_cas_pvr_try(uint32_t pvr) "0x%x"
> > > diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> > > index 518dd06e98..aac5667bf4 100644
> > > --- a/target/ppc/kvm.c
> > > +++ b/target/ppc/kvm.c
> > > @@ -2456,6 +2456,11 @@ bool kvmppc_has_cap_mmu_hash_v3(void)
> > > return cap_mmu_hash_v3;
> > > }
> > >
> > > +int kvmppc_cap_smt_possible(void)
> > > +{
> > > + return cap_ppc_smt_possible;
> > > +}
> > > +
> > > PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)
> > > {
> > > uint32_t host_pvr = mfpvr();
> > > diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> > > index ecb55493cc..6ac33d2b4a 100644
> > > --- a/target/ppc/kvm_ppc.h
> > > +++ b/target/ppc/kvm_ppc.h
> > > @@ -59,6 +59,7 @@ bool kvmppc_has_cap_fixup_hcalls(void);
> > > bool kvmppc_has_cap_htm(void);
> > > bool kvmppc_has_cap_mmu_radix(void);
> > > bool kvmppc_has_cap_mmu_hash_v3(void);
> > > +int kvmppc_cap_smt_possible(void);
> > > int kvmppc_enable_hwrng(void);
> > > int kvmppc_put_books_sregs(PowerPCCPU *cpu);
> > > PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void);
> > > @@ -290,6 +291,11 @@ static inline bool kvmppc_has_cap_mmu_hash_v3(void)
> > > return false;
> > > }
> > >
> > > +static inline int kvmppc_cap_smt_possible(void)
> > > +{
> > > + return -1;
> >
> > When CONFIG_KVM is set, the semantics of kvmppc_cap_smt_possible() is:
> > - a bitmap with supported SMT modes if KVM has KVM_CAP_PPC_SMT_POSSIBLE
> > - 0 if KVM doesn't have KVM_CAP_PPC_SMT_POSSIBLE or we're running in
> > TCG mode
> >
> > so it looks a bit weird to return -1 when CONFIG_KVM isn't set (when
> > running in TCG mode, we would get different values depending on how
> > the QEMU binary was compiled).
> >
> > Shouldn't this stub return 0 instead ?
>
> YES! it *must* be otherwise TCG would accept any smt mode,
> I'll change it.
>
> Thanks :-)
>
> >
> > Cheers,
> >
> > --
> > Greg
> >
> > > +}
> > > +
> > > static inline int kvmppc_enable_hwrng(void)
> > > {
> > > return -1;
> >
>