On Fri, 5 Jan 2018 22:47:22 -0200
Jose Ricardo Ziviani <jos...@linux.vnet.ibm.com> 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 <sathe...@in.ibm.com>
> Signed-off-by: Jose Ricardo Ziviani <jos...@linux.vnet.ibm.com>
> ---
Hi,
I agree with the general idea but I have a few questions.
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 ?
> 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...
> +
> + /* 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 ?
>
> 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 ?
Cheers,
--
Greg
> +}
> +
> static inline int kvmppc_enable_hwrng(void)
> {
> return -1;
