linux-kernel - Re: [PATCH RESEND v2 1/8] perf/x86/core: Completely disable guest PEBS via guest's global

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Date:   Tue, 30 Aug 2022 17:40:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH RESEND v2 1/8] perf/x86/core: Completely disable guest
 PEBS via guest's global_ctrl

On Tue, Aug 23, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> When a guest PEBS counter is cross-mapped by a host counter, software
> will remove the corresponding bit in the arr[global_ctrl].guest and
> expect hardware to perform a change of state "from enable to disable"
> via the msr_slot[] switch during the vmx transaction.
> 
> The real world is that if user adjust the counter overflow value small
> enough, it still opens a tiny race window for the previously PEBS-enabled
> counter to write cross-mapped PEBS records into the guest's PEBS buffer,
> when arr[global_ctrl].guest has been prioritised (switch_msr_special stuff)
> to switch into the enabled state, while the arr[pebs_enable].guest has not.
> 
> Close this window by clearing invalid bits in the arr[global_ctrl].guest.
> 
> Fixes: 854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/events/intel/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 2db93498ff71..75cdd11ab014 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4052,8 +4052,9 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>  		/* Disable guest PEBS if host PEBS is enabled. */
>  		arr[pebs_enable].guest = 0;
>  	} else {
> -		/* Disable guest PEBS for cross-mapped PEBS counters. */
> +		/* Disable guest PEBS thoroughly for cross-mapped PEBS counters. */
>  		arr[pebs_enable].guest &= ~kvm_pmu->host_cross_mapped_mask;
> +		arr[global_ctrl].guest &= ~kvm_pmu->host_cross_mapped_mask;
>  		/* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
>  		arr[global_ctrl].guest |= arr[pebs_enable].guest;
>  	}

Please post this as a separate patch to the perf folks (Cc: kvm@).