Re: [PATCH] KVM: x86/pmu: Isolate TSX specific perf_event_attr.attr logic for AMD

[Jim Mattson]

On Tue, Mar 8, 2022 at 3:59 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
>
> On 8/3/2022 5:39 am, Jim Mattson wrote:
> > On Sun, Mar 6, 2022 at 10:38 PM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
> >>
> >> From: Like Xu <likexu@xxxxxxxxxxx>
> >>
> >> HSW_IN_TX* bits are used in generic code which are not supported on
> >> AMD. Worse, these bits overlap with AMD EventSelect[11:8] and hence
> >> using HSW_IN_TX* bits unconditionally in generic code is resulting in
> >> unintentional pmu behavior on AMD. For example, if EventSelect[11:8]
> >> is 0x2, pmc_reprogram_counter() wrongly assumes that
> >> HSW_IN_TX_CHECKPOINTED is set and thus forces sampling period to be 0.
> >>
> >> Opportunistically remove two TSX specific incoming parameters for
> >> the generic interface reprogram_counter().
> >>
> >> Fixes: 103af0a98788 ("perf, kvm: Support the in_tx/in_tx_cp modifiers in KVM arch perfmon emulation v5")
> >> Co-developed-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> >> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> >> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> >> ---
> >> Note: this patch is based on [1] which is considered to be a necessary cornerstone.
> >> [1] https://lore.kernel.org/kvm/20220302111334.12689-1-likexu@xxxxxxxxxxx/
> >>
> >>   arch/x86/kvm/pmu.c | 29 ++++++++++++++---------------
> >>   1 file changed, 14 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> >> index 17c61c990282..d0f9515c37dd 100644
> >> --- a/arch/x86/kvm/pmu.c
> >> +++ b/arch/x86/kvm/pmu.c
> >> @@ -99,8 +99,7 @@ static void kvm_perf_overflow(struct perf_event *perf_event,
> >>
> >>   static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
> >>                                    u64 config, bool exclude_user,
> >> -                                 bool exclude_kernel, bool intr,
> >> -                                 bool in_tx, bool in_tx_cp)
> >> +                                 bool exclude_kernel, bool intr)
> >>   {
> >>          struct perf_event *event;
> >>          struct perf_event_attr attr = {
> >> @@ -116,16 +115,18 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
> >>
> >>          attr.sample_period = get_sample_period(pmc, pmc->counter);
> >>
> >> -       if (in_tx)
> >> -               attr.config |= HSW_IN_TX;
> >> -       if (in_tx_cp) {
> >> -               /*
> >> -                * HSW_IN_TX_CHECKPOINTED is not supported with nonzero
> >> -                * period. Just clear the sample period so at least
> >> -                * allocating the counter doesn't fail.
> >> -                */
> >> -               attr.sample_period = 0;
> >> -               attr.config |= HSW_IN_TX_CHECKPOINTED;
> >> +       if (guest_cpuid_is_intel(pmc->vcpu)) {
> >
> > This is not the right condition to check. Per the SDM, both bits 32
> > and 33 "may only be set if the processor supports HLE or RTM." On
> > other Intel processors, this bit is reserved and any attempts to set
> > them result in a #GP.
>
> We already have this part of the code:
>
>         entry = kvm_find_cpuid_entry(vcpu, 7, 0);
>         if (entry &&
>             (boot_cpu_has(X86_FEATURE_HLE) || boot_cpu_has(X86_FEATURE_RTM)) &&
>             (entry->ebx & (X86_FEATURE_HLE|X86_FEATURE_RTM)))
>                 pmu->reserved_bits ^= HSW_IN_TX|HSW_IN_TX_CHECKPOINTED;

I stand corrected.

> > additional constraint which is ignored here: "This bit may only be set
> > for IA32_PERFEVTSEL2." I have confirmed that a #GP is raised for an
> > attempt to set bit 33 in any PerfEvtSeln other than PerfEvtSel2 on a
> > Broadwell Xeon E5.
>
> Yes, "19.3.6.5 Performance Monitoring and Intel® TSX".
>
> I'm not sure if the host perf scheduler indicate this restriction.

Whether it does or it doesn't, it's KVM's responsibility to synthesize
the #GP if IN_TXCP is set for any PerfEvtSeln other than PerfEvtSel2.