Re: [PATCH v4 07/15] RISC-V: KVM: No need to exit to the user space if perf event failed

[Atish Patra]

On Sat, Mar 2, 2024 at 12:16 AM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Feb 28, 2024 at 05:01:22PM -0800, Atish Patra wrote:
> > Currently, we return a linux error code if creating a perf event failed
> > in kvm. That shouldn't be necessary as guest can continue to operate
> > without perf profiling or profiling with firmware counters.
> >
> > Return appropriate SBI error code to indicate that PMU configuration
> > failed. An error message in kvm already describes the reason for failure.
>
> I don't know enough about the perf subsystem to know if there may be
> a concern that resources are temporarily unavailable. If so, then this

Do you mean the hardware resources unavailable because the host is using it ?

> patch would make it possible for a guest to do the exact same thing,
> but sometimes succeed and sometimes get SBI_ERR_NOT_SUPPORTED.
> sbi_pmu_counter_config_matching doesn't currently have any error types
> specified that say "unsupported at the moment, maybe try again", which
> would be more appropriate in that case. I do see
> perf_event_create_kernel_counter() can return ENOMEM when memory isn't
> available, but if the kernel isn't able to allocate a small amount of
> memory, then we're in bigger trouble anyway, so the concern would be
> if there are perf resource pools which may temporarily be exhausted at
> the time the guest makes this request.
>

For other cases, this patch ensures that guests continue to run without failure
which allows the user in the guest to try again if this fails due to a temporary
resource availability.

> One comment below.
>
> >
> > Fixes: 0cb74b65d2e5 ("RISC-V: KVM: Implement perf support without sampling")
> > Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>
> > Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> > ---
> >  arch/riscv/kvm/vcpu_pmu.c     | 14 +++++++++-----
> >  arch/riscv/kvm/vcpu_sbi_pmu.c |  6 +++---
> >  2 files changed, 12 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
> > index b1574c043f77..29bf4ca798cb 100644
> > --- a/arch/riscv/kvm/vcpu_pmu.c
> > +++ b/arch/riscv/kvm/vcpu_pmu.c
> > @@ -229,8 +229,9 @@ static int kvm_pmu_validate_counter_mask(struct kvm_pmu *kvpmu, unsigned long ct
> >       return 0;
> >  }
> >
> > -static int kvm_pmu_create_perf_event(struct kvm_pmc *pmc, struct perf_event_attr *attr,
> > -                                  unsigned long flags, unsigned long eidx, unsigned long evtdata)
> > +static long kvm_pmu_create_perf_event(struct kvm_pmc *pmc, struct perf_event_attr *attr,
> > +                                   unsigned long flags, unsigned long eidx,
> > +                                   unsigned long evtdata)
> >  {
> >       struct perf_event *event;
> >
> > @@ -454,7 +455,8 @@ int kvm_riscv_vcpu_pmu_ctr_cfg_match(struct kvm_vcpu *vcpu, unsigned long ctr_ba
> >                                    unsigned long eidx, u64 evtdata,
> >                                    struct kvm_vcpu_sbi_return *retdata)
> >  {
> > -     int ctr_idx, ret, sbiret = 0;
> > +     int ctr_idx, sbiret = 0;
> > +     long ret;
> >       bool is_fevent;
> >       unsigned long event_code;
> >       u32 etype = kvm_pmu_get_perf_event_type(eidx);
> > @@ -513,8 +515,10 @@ int kvm_riscv_vcpu_pmu_ctr_cfg_match(struct kvm_vcpu *vcpu, unsigned long ctr_ba
> >                       kvpmu->fw_event[event_code].started = true;
> >       } else {
> >               ret = kvm_pmu_create_perf_event(pmc, &attr, flags, eidx, evtdata);
> > -             if (ret)
> > -                     return ret;
> > +             if (ret) {
> > +                     sbiret = SBI_ERR_NOT_SUPPORTED;
> > +                     goto out;
> > +             }
> >       }
> >
> >       set_bit(ctr_idx, kvpmu->pmc_in_use);
> > diff --git a/arch/riscv/kvm/vcpu_sbi_pmu.c b/arch/riscv/kvm/vcpu_sbi_pmu.c
> > index 7eca72df2cbd..b70179e9e875 100644
> > --- a/arch/riscv/kvm/vcpu_sbi_pmu.c
> > +++ b/arch/riscv/kvm/vcpu_sbi_pmu.c
> > @@ -42,9 +42,9 @@ static int kvm_sbi_ext_pmu_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
> >  #endif
> >               /*
> >                * This can fail if perf core framework fails to create an event.
> > -              * Forward the error to userspace because it's an error which
> > -              * happened within the host kernel. The other option would be
> > -              * to convert to an SBI error and forward to the guest.
> > +              * No need to forward the error to userspace and exit the guest
>
> Period after guest
>
>
> > +              * operation can continue without profiling. Forward the
>
> The operation
>

Fixed the above two.


> > +              * appropriate SBI error to the guest.
> >                */
> >               ret = kvm_riscv_vcpu_pmu_ctr_cfg_match(vcpu, cp->a0, cp->a1,
> >                                                      cp->a2, cp->a3, temp, retdata);
> > --
> > 2.34.1
> >
>
> Thanks,
> drew



--
Regards,
Atish