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Peter Anvin" , Andrii Nakryiko , Song Liu , Ravi Bangoria , Ananth Narayan Subject: Re: [PATCH] perf/x86/amd/lbr: Limit branch snapshots to valid entries Message-ID: <20261013074625.GC2009045@noisy.programming.kicks-ass.net> References: <20261007181002.4005262-1-sandipan.das@amd.com> <20261012153014.GU4121339@noisy.programming.kicks-ass.net> <94d387f2-ff07-4895-a722-e8ded4203309@amd.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <94d387f2-ff07-4895-a722-e8ded4203309@amd.com> On Tue, Oct 13, 2026 at 12:14:58PM +0530, Sandipan Das wrote: > On 12-10-2026 21:00, Peter Zijlstra wrote: > > On Wed, Oct 07, 2026 at 11:40:02PM +0530, Sandipan Das wrote: > >> The branch snapshot callback used by bpf_get_branch_snapshot() copies > >> up to x86_pmu.lbr_nr entries from cpuc->lbr_entries[], even when the > >> last LBR read kept fewer branches. The entries past lbr_stack.nr are > >> invalid, but they end up reaching BPF programs. > >> > >> Limit copying to lbr_stack.nr entries and also reset lbr_stack.nr when > >> amd_pmu_lbr_read() skips reading the registers, so that a stale count > >> is never reused. > >> > >> Fixes: a4d18112e531 ("perf/x86/amd: Support capturing LBR from software events") > >> Reported-by: Sashiko > >> Closes: https://lore.kernel.org/all/sashiko-outbox-163130@kernel.org/ > >> Signed-off-by: Sandipan Das > >> --- > >> arch/x86/events/amd/core.c | 2 +- > >> arch/x86/events/amd/lbr.c | 4 +++- > >> 2 files changed, 4 insertions(+), 2 deletions(-) > >> > >> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c > >> index 49b6b8fce566..bf631f60757c 100644 > >> --- a/arch/x86/events/amd/core.c > >> +++ b/arch/x86/events/amd/core.c > >> @@ -944,7 +944,7 @@ static int amd_pmu_v2_snapshot_branch_stack(struct perf_branch_entry *entries, u > >> cpuc = this_cpu_ptr(&cpu_hw_events); > >> > >> amd_pmu_lbr_read(); > >> - cnt = min(cnt, x86_pmu.lbr_nr); > >> + cnt = min(cnt, cpuc->lbr_stack.nr); > >> memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt); > >> > >> amd_pmu_v2_enable_all(0); > >> diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c > >> index 1e1b2f08cb51..a00d3850c4a8 100644 > >> --- a/arch/x86/events/amd/lbr.c > >> +++ b/arch/x86/events/amd/lbr.c > >> @@ -165,8 +165,10 @@ void amd_pmu_lbr_read(void) > >> struct branch_entry entry; > >> int out = 0, idx, i; > >> > >> - if (!cpuc->lbr_users) > >> + if (!cpuc->lbr_users) { > >> + cpuc->lbr_stack.nr = 0; > >> return; > >> + } > >> > >> for (i = 0; i < x86_pmu.lbr_nr; i++) { > >> entry.from.full = amd_pmu_lbr_get_from(i); > > > > You now seem to deviate from what the Intel code does. Please clarify. > > Yes, it is a deviation. I thought of proposing the following but didn't > proceed because I was not sure about the if condition in the second hunk > which considers things other than cpuc->lbr_users for an early return. Dapeng, can you look into this? I would really like to keep behaviour consistent where possible. > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c > index 0a34d674df59..5dfb9e9156e8 100644 > --- a/arch/x86/events/intel/core.c > +++ b/arch/x86/events/intel/core.c > @@ -2822,7 +2822,7 @@ __intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, > struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events); > > intel_pmu_lbr_read(); > - cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr); > + cnt = min_t(unsigned int, cnt, cpuc->lbr_stack.nr); > > memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt); > intel_pmu_enable_all(0); > diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c > index 0b446d8912e4..6b070490f461 100644 > --- a/arch/x86/events/intel/lbr.c > +++ b/arch/x86/events/intel/lbr.c > @@ -1026,8 +1026,10 @@ void intel_pmu_lbr_read(void) > * but this simple approach seems to work for now. > */ > if (!cpuc->lbr_users || vlbr_exclude_host() || > - cpuc->lbr_users == cpuc->lbr_pebs_users) > + cpuc->lbr_users == cpuc->lbr_pebs_users) { > + cpuc->lbr_stack.nr = 0; > return; > + } > > x86_pmu.lbr_read(cpuc); >