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

[Andrew Jones]

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
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.

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

> +		 * 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