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Message-ID: <ZedUwKWW7PNkvUH1@google.com>
Date: Tue, 5 Mar 2024 09:22:08 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Like Xu <like.xu.linux@...il.com>
Cc: Dapeng Mi <dapeng1.mi@...ux.intel.com>, Sandipan Das <sandipan.das@....com>,
pbonzini@...hat.com, mizhang@...gle.com, jmattson@...gle.com,
ravi.bangoria@....com, nikunj.dadhania@....com, santosh.shukla@....com,
manali.shukla@....com, babu.moger@....com, kvm list <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/svm/pmu: Set PerfMonV2 global control bits correctly
On Tue, Mar 05, 2024, Like Xu wrote:
> On 5/3/2024 3:46 am, Sean Christopherson wrote:
> > > > > > ---
> > > > > > arch/x86/kvm/svm/pmu.c | 1 +
> > > > > > 1 file changed, 1 insertion(+)
> > > > > >
> > > > > > diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> > > > > > index b6a7ad4d6914..14709c564d6a 100644
> > > > > > --- a/arch/x86/kvm/svm/pmu.c
> > > > > > +++ b/arch/x86/kvm/svm/pmu.c
> > > > > > @@ -205,6 +205,7 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> > > > > > if (pmu->version > 1) {
> > > > > > pmu->global_ctrl_mask = ~((1ull << pmu->nr_arch_gp_counters) - 1);
> > > > > > pmu->global_status_mask = pmu->global_ctrl_mask;
> > > > > > + pmu->global_ctrl = ~pmu->global_ctrl_mask;
> > >
> > > It seems to be more easily understand to calculate global_ctrl firstly and
> > > then derive the globol_ctrl_mask (negative logic).
> >
> > Hrm, I'm torn. On one hand, awful name aside (global_ctrl_mask should really be
> > something like global_ctrl_rsvd_bits), the computation of the reserved bits should
> > come from the capabilities of the PMU, not from the RESET value.
> >
> > On the other hand, setting _all_ non-reserved bits will likely do the wrong thing
> > if AMD ever adds bits in PerfCntGlobalCtl that aren't tied to general purpose
> > counters. But, that's a future theoretical problem, so I'm inclined to vote for
> > Sandipan's approach.
>
> I suspect that Intel hardware also has this behaviour [*] although guest
> kernels using Intel pmu version 1 are pretty much non-existent.
>
> [*] Table 10-1. IA-32 and Intel® 64 Processor States Following Power-up,
> Reset, or INIT (Contd.)
Aha! Nice. To save people lookups, the table says:
IA32_PERF_GLOBAL_CTRL: Sets bits n-1:0 and clears the upper bits.
and
Where "n" is the number of general-purpose counters available in the processor.
Which means that (a) KVM can handle this in common code and (b) we can dodge the
whole reserved bits chicken-and-egg problem since global_ctrl *can't* be derived
from global_ctrl_mask.
This? (compile tested only)
---
From: Sean Christopherson <seanjc@...gle.com>
Date: Tue, 5 Mar 2024 09:02:26 -0800
Subject: [PATCH] KVM: x86/pmu: Set enable bits for GP counters in
PERF_GLOBAL_CTRL at "RESET"
Set the enable bits for general purpose counters in IA32_PERF_GLOBAL_CTRL
when refreshing the PMU to emulate the MSR's architecturally defined
post-RESET behavior. Per Intel's SDM:
IA32_PERF_GLOBAL_CTRL: Sets bits n-1:0 and clears the upper bits.
and
Where "n" is the number of general-purpose counters available in the processor.
This is a long-standing bug that was recently exposed when KVM added
supported for AMD's PerfMonV2, i.e. when KVM started exposing a vPMU with
PERF_GLOBAL_CTRL to guest software that only knew how to program v1 PMUs
(that don't support PERF_GLOBAL_CTRL). Failure to emulate the post-RESET
behavior results in such guests unknowingly leaving all general purpose
counters globally disabled (the entire reason the post-RESET value sets
the GP counter enable bits is to maintain backwards compatibility).
The bug has likely gone unnoticed because PERF_GLOBAL_CTRL has been
supported on Intel CPUs for as long as KVM has existed, i.e. hardly anyone
is running guest software that isn't aware of PERF_GLOBAL_CTRL on Intel
PMUs.
Note, kvm_pmu_refresh() can be invoked multiple times, i.e. it's not a
"pure" RESET flow. But it can only be called prior to the first KVM_RUN,
i.e. the guest will only ever observe the final value.
Reported-by: Reported-by: Babu Moger <babu.moger@....com>
Cc: Like Xu <like.xu.linux@...il.com>
Cc: Mingwei Zhang <mizhang@...gle.com>
Cc: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Sandipan Das <sandipan.das@....com>
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/kvm/pmu.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 87cc6c8809ad..f61ce26aeb90 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -741,6 +741,8 @@ static void kvm_pmu_reset(struct kvm_vcpu *vcpu)
*/
void kvm_pmu_refresh(struct kvm_vcpu *vcpu)
{
+ struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
+
if (KVM_BUG_ON(kvm_vcpu_has_run(vcpu), vcpu->kvm))
return;
@@ -750,8 +752,18 @@ void kvm_pmu_refresh(struct kvm_vcpu *vcpu)
*/
kvm_pmu_reset(vcpu);
- bitmap_zero(vcpu_to_pmu(vcpu)->all_valid_pmc_idx, X86_PMC_IDX_MAX);
+ bitmap_zero(pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
static_call(kvm_x86_pmu_refresh)(vcpu);
+
+ /*
+ * At RESET, both Intel and AMD CPUs set all enable bits for general
+ * purpose counters in IA32_PERF_GLOBAL_CTRL (so that software that
+ * was written for v1 PMUs don't unknowingly leave GP counters disabled
+ * in the global controls). Emulate that behavior when refreshing the
+ * PMU so that userspace doesn't need to manually set PERF_GLOBAL_CTRL.
+ */
+ if (kvm_pmu_has_perf_global_ctrl(pmu))
+ pmu->global_ctrl = GENMASK_ULL(pmu->nr_arch_gp_counters - 1, 0);
}
void kvm_pmu_init(struct kvm_vcpu *vcpu)
base-commit: 1d7ae977d219e68698fdb9bed1049dc561038aa1
--
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