linux-kernel - Re: [PATCH 13/13] x86: retpolines: eliminate retpoline from msr event handlers

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Message-ID: <306784c2-25b0-9f25-c9e0-c23aaee11e4f@redhat.com>
Date:   Tue, 5 Nov 2019 11:21:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 13/13] x86: retpolines: eliminate retpoline from msr event
 handlers

On 05/11/19 00:00, Andrea Arcangeli wrote:
> It's enough to check the value and issue the direct call.
> 
> After this commit is applied, here the most common retpolines executed
> under a high resolution timer workload in the guest on a VMX host:
> 
> [..]
> @[
>     trace_retpoline+1
>     __trace_retpoline+30
>     __x86_indirect_thunk_rax+33
>     do_syscall_64+89
>     entry_SYSCALL_64_after_hwframe+68
> ]: 267
> @[]: 2256
> @[
>     trace_retpoline+1
>     __trace_retpoline+30
>     __x86_indirect_thunk_rax+33
>     __kvm_wait_lapic_expire+284
>     vmx_vcpu_run.part.97+1091
>     vcpu_enter_guest+377
>     kvm_arch_vcpu_ioctl_run+261
>     kvm_vcpu_ioctl+559
>     do_vfs_ioctl+164
>     ksys_ioctl+96
>     __x64_sys_ioctl+22
>     do_syscall_64+89
>     entry_SYSCALL_64_after_hwframe+68
> ]: 2390
> @[]: 33410
> 
> @total: 315707
> 
> Note the highest hit above is __delay so probably not worth optimizing
> even if it would be more frequent than 2k hits per sec.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  arch/x86/events/intel/core.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index fcef678c3423..937363b803c1 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -3323,8 +3323,19 @@ static int intel_pmu_hw_config(struct perf_event *event)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_RETPOLINE
> +static struct perf_guest_switch_msr *core_guest_get_msrs(int *nr);
> +static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr);
> +#endif
> +
>  struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
>  {
> +#ifdef CONFIG_RETPOLINE
> +	if (x86_pmu.guest_get_msrs == intel_guest_get_msrs)
> +		return intel_guest_get_msrs(nr);
> +	else if (x86_pmu.guest_get_msrs == core_guest_get_msrs)
> +		return core_guest_get_msrs(nr);
> +#endif
>  	if (x86_pmu.guest_get_msrs)
>  		return x86_pmu.guest_get_msrs(nr);
>  	*nr = 0;
> 

Queued, thanks.

Paolo