Re: [kvm-unit-tests PATCH 4/4] x86/pmu: Add a PEBS test to verify the host LBRs aren't leaked to the guest

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 3/7/2024 7:01 AM, Sean Christopherson wrote:
> When using adaptive PEBS with LBR entries, verify that the LBR entries are
> all '0'.  If KVM fails to context switch LBRs, e.g. when the guest isn't
> using LBRs, as is the case in the pmu_pebs test, then adaptive PEBS can be
> used to read the *host* LBRs as the CPU doesn't enforce the VMX MSR bitmaps
> when generating PEBS records, i.e. ignores KVM's interception of reads to
> LBR MSRs.
>
> This testcase is best exercised by simultaneously utilizing LBRs in the
> host, e.g. by running "perf record -b -e instructions", so that there is
> non-zero data in the LBR MSRs.
>
> Cc: Like Xu <like.xu.linux@gmail.com>
> Cc: Mingwei Zhang <mizhang@google.com>
> Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
> Cc: Zhang Xiong <xiong.y.zhang@intel.com>
> Cc: Lv Zhiyuan <zhiyuan.lv@intel.com>
> Cc: Dapeng Mi <dapeng1.mi@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   x86/pmu_pebs.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/x86/pmu_pebs.c b/x86/pmu_pebs.c
> index 0e8d60c3..df8e736f 100644
> --- a/x86/pmu_pebs.c
> +++ b/x86/pmu_pebs.c
> @@ -299,6 +299,19 @@ static void check_pebs_records(u64 bitmask, u64 pebs_data_cfg, bool use_adaptive
>   		expected = pebs_idx_match && pebs_size_match && data_cfg_match;
>   		report(expected,
>   		       "PEBS record (written seq %d) is verified (including size, counters and cfg).", count);
> +		if (use_adaptive && (pebs_data_cfg & PEBS_DATACFG_LBRS)) {
> +			unsigned int lbrs_offset = get_pebs_record_size(pebs_data_cfg & ~PEBS_DATACFG_LBRS, true);
> +			struct lbr_entry *pebs_lbrs = cur_record + lbrs_offset;
> +			int i;
> +
> +			for (i = 0; i < MAX_NUM_LBR_ENTRY; i++) {
> +				if (!pebs_lbrs[i].from && !pebs_lbrs[i].to)
> +					continue;
> +
> +				report_fail("PEBS LBR record %u isn't empty, got from = '%lx', to = '%lx', info = '%lx'",
> +					    i, pebs_lbrs[i].from, pebs_lbrs[i].to, pebs_lbrs[i].info);
> +			}
> +		}
>   		cur_record = cur_record + pebs_record_size;
>   		count++;
>   	} while (expected && (void *)cur_record < (void *)ds->pebs_index);
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>