linux-kernel - Re: [PATCH] KVM: x86/pmu: Prevent any host user from enabling PEBS for profiling guest

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Date:   Wed, 29 Nov 2023 09:38:27 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: Prevent any host user from enabling PEBS
 for profiling guest



On 2023-11-29 4:50 a.m., Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> Stop using PEBS counters on host to profiling guest. Limit the range of
> enabled PEBS counters to only those counters enabled from the guest PEBS
> emulation perspective.
> 
> If there is a perf-record agent on host that uses perf-tools events like
> "cpu-cycles:GP" (G for attr.exclude_host, P for max precise event counter)
> to capture guest performance events, then the guest will be hanged. This is
> because Intel DS-based PEBS buffer is addressed using the 64-bit linear
> address of the current {p/v}CPU context based on MSR_IA32_DS_AREA.
> 
> Any perf user using PEBS counters to profile guest on host is, in perf/core
> implementation details, trying to set bits on cpuc->intel_ctrl_guest_mask
> and arr[pebs_enable].guest, much like the guest PEBS emulation behaviour.
> But the subsequent PEBS memory write, regardless of whether guest PEBS is
> enabled, can overshoot guest entry and corrupt guest memory.
> 
> Profiling guest via PEBS-DS buffer on host is not supported at this time.
> Fix this by filtering the real configured value of arr[pebs_enable].guest
> with the emulated state of guest enabled PEBS counters, under the condition
> of none cross-mapped PEBS counters.

So the counter will be silently disabled. The user never knows why
nothing is sampled.
Since we don't support the case, profiling guest via PEBS-DS buffer on
host. Maybe we should error out when creating the event. For example
(not tested),

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 3871267d3237..24b90c70737f 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3958,6 +3958,10 @@ static int intel_pmu_hw_config(struct perf_event
*event)
 		if ((event->attr.config & INTEL_ARCH_EVENT_MASK) ==
INTEL_FIXED_VLBR_EVENT)
 			return -EINVAL;

+		/* Profiling guest via PEBS-DS buffer on host is not supported. */
+		if (event->attr.exclude_host)
+			return -EINVAL;
+
 		if (!(event->attr.freq || (event->attr.wakeup_events &&
!event->attr.watermark))) {
 		    event->hw.flags |= PERF_X86_EVENT_AUTO_RELOAD;
 			if (!(event->attr.sample_type &


Thanks,
Kan

> 
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/events/intel/core.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index a08f794a0e79..17afd504c35b 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4103,13 +4103,19 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>  		.guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
>  	};
>  
> +	/* In any case, clear guest PEBS bits first. */
> +	arr[global_ctrl].guest &= ~arr[pebs_enable].guest;
> +
>  	if (arr[pebs_enable].host) {
>  		/* Disable guest PEBS if host PEBS is enabled. */
>  		arr[pebs_enable].guest = 0;
>  	} else {
>  		/* 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;
> +
> +		/* Prevent any host user from enabling PEBS for profiling guest. */
> +		arr[pebs_enable].guest &= (kvm_pmu->pebs_enable & kvm_pmu->global_ctrl);
> +
>  		/* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
>  		arr[global_ctrl].guest |= arr[pebs_enable].guest;
>  	}
> 
> base-commit: 6803fb00772cc50cd59a66bd8caaee5c84b13fcf