Re: [PATCH] KVM: x86/pmu: Add Intel PMU supported fixed counters bit mask

From: Like Xu
Date: Mon Mar 27 2023 - 03:48:14 EST

On 25/3/2023 7:19 am, Jim Mattson wrote:
On Tue, Mar 21, 2023 at 4:28 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:

From: Like Xu <likexu@xxxxxxxxxxx>

Per Intel SDM, fixed-function performance counter 'i' is supported if:

FxCtr[i]_is_supported := ECX[i] || (EDX[4:0] > i);

which means that the KVM user space can use EDX to limit the number of
fixed counters and at the same time, using ECX to enable part of other
KVM supported fixed counters.

Add a bitmap (instead of always checking the vcpu's CPUIDs) to keep track
of the guest available fixed counters and perform the semantic checks.

Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/pmu.h | 8 +++++
arch/x86/kvm/vmx/pmu_intel.c | 53 +++++++++++++++++++++------------
3 files changed, 44 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index a45de1118a42..14689e583127 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -565,6 +565,8 @@ struct kvm_pmu {
*/
bool need_cleanup;

+ DECLARE_BITMAP(supported_fixed_pmc_idx, KVM_PMC_MAX_FIXED);
+
/*
* The total number of programmed perf_events and it helps to avoid
* redundant check before cleanup if guest don't use vPMU at all.
diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index be62c16f2265..9f4504e5e9d5 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -111,6 +111,11 @@ static inline struct kvm_pmc *get_gp_pmc(struct kvm_pmu *pmu, u32 msr,
return NULL;
}

+static inline bool fixed_ctr_is_supported(struct kvm_pmu *pmu, unsigned int idx)
+{
+ return test_bit(idx, pmu->supported_fixed_pmc_idx);
+}
+
/* returns fixed PMC with the specified MSR */
static inline struct kvm_pmc *get_fixed_pmc(struct kvm_pmu *pmu, u32 msr)
{
@@ -120,6 +125,9 @@ static inline struct kvm_pmc *get_fixed_pmc(struct kvm_pmu *pmu, u32 msr)
u32 index = array_index_nospec(msr - base,
pmu->nr_arch_fixed_counters);

+ if (!fixed_ctr_is_supported(pmu, index))
+ return NULL;
+
return &pmu->fixed_counters[index];
}

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index e8a3be0b9df9..12f4b2fe7756 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -43,13 +43,16 @@ static int fixed_pmc_events[] = {1, 0, 7};
static void reprogram_fixed_counters(struct kvm_pmu *pmu, u64 data)
{
struct kvm_pmc *pmc;
- u8 old_fixed_ctr_ctrl = pmu->fixed_ctr_ctrl;
+ u8 new_ctrl, old_ctrl, old_fixed_ctr_ctrl = pmu->fixed_ctr_ctrl;
int i;

pmu->fixed_ctr_ctrl = data;
for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
- u8 new_ctrl = fixed_ctrl_field(data, i);
- u8 old_ctrl = fixed_ctrl_field(old_fixed_ctr_ctrl, i);
+ if (!fixed_ctr_is_supported(pmu, i))
+ continue;
+
+ new_ctrl = fixed_ctrl_field(data, i);
+ old_ctrl = fixed_ctrl_field(old_fixed_ctr_ctrl, i);

if (old_ctrl == new_ctrl)
continue;
@@ -125,6 +128,9 @@ static bool intel_is_valid_rdpmc_ecx(struct kvm_vcpu *vcpu, unsigned int idx)

idx &= ~(3u << 30);

+ if (fixed && !fixed_ctr_is_supported(pmu, idx))
+ return false;
+
return fixed ? idx < pmu->nr_arch_fixed_counters
: idx < pmu->nr_arch_gp_counters;
}
@@ -145,7 +151,7 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
counters = pmu->gp_counters;
num_counters = pmu->nr_arch_gp_counters;
}
- if (idx >= num_counters)
+ if (idx >= num_counters || (fixed && !fixed_ctr_is_supported(pmu, idx)))
return NULL;
*mask &= pmu->counter_bitmask[fixed ? KVM_PMC_FIXED : KVM_PMC_GP];
return &counters[array_index_nospec(idx, num_counters)];
@@ -500,6 +506,9 @@ static void setup_fixed_pmc_eventsel(struct kvm_pmu *pmu)
int i;

for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
+ if (!fixed_ctr_is_supported(pmu, i))
+ continue;
+
pmc = &pmu->fixed_counters[i];
event = fixed_pmc_events[array_index_nospec(i, size)];
pmc->eventsel = (intel_arch_events[event].unit_mask << 8) |
@@ -520,6 +529,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)

pmu->nr_arch_gp_counters = 0;
pmu->nr_arch_fixed_counters = 0;
+ bitmap_zero(pmu->supported_fixed_pmc_idx, KVM_PMC_MAX_FIXED);
pmu->counter_bitmask[KVM_PMC_GP] = 0;
pmu->counter_bitmask[KVM_PMC_FIXED] = 0;
pmu->version = 0;
@@ -551,13 +561,24 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
pmu->available_event_types = ~entry->ebx &
((1ull << eax.split.mask_length) - 1);

- if (pmu->version == 1) {
- pmu->nr_arch_fixed_counters = 0;
- } else {
+ counter_mask = ~(BIT_ULL(pmu->nr_arch_gp_counters) - 1);
+ bitmap_set(pmu->all_valid_pmc_idx, 0, pmu->nr_arch_gp_counters);
+
+ if (pmu->version > 1) {
pmu->nr_arch_fixed_counters =
- min3(ARRAY_SIZE(fixed_pmc_events),
- (size_t) edx.split.num_counters_fixed,
- (size_t)kvm_pmu_cap.num_counters_fixed);
+ min_t(int, ARRAY_SIZE(fixed_pmc_events),
+ kvm_pmu_cap.num_counters_fixed);
+ for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
+ /* FxCtr[i]_is_supported := CPUID.0xA.ECX[i] || (EDX[4:0] > i) */

This is true only when pmu->version >= 5.

This is true in for "Version 5" section, but not mentioned in the CPUID.0xA section.
I would argue that this is a deliberate omission for the instruction implementation,
as it does use the word "version>1" in the near CPUID.0xA.EDX section.

For virtualised use, this feature offers a kind of flexibility as users can enable part of
the fixed counters, don't you think? Or maybe you're more looking forward to the
patch set that raises the vPMU version number from 2 to 5, that part of the code
was already in my tree some years ago.


From the SDM, volume 3, section 20.2.5 Architectural Performance
Monitoring Version 5:

With Architectural Performance Monitoring Version 5, register
CPUID.0AH.ECX indicates Fixed Counter enumeration. It is a bit mask
which enumerates the supported Fixed Counters in a processor. If bit
'i' is set, it implies that Fixed Counter 'i' is supported. Software
is recommended to use the following logic to check if a Fixed Counter
is supported on a given processor: FxCtr[i]_is_supported := ECX[i] ||
(EDX[4:0] > i);

Prior to PMU version 5, all fixed counters from 0 through <number of
fixed counters - 1> are supported.

+ if (!(entry->ecx & BIT_ULL(i) ||
+ edx.split.num_counters_fixed > i))
+ continue;
+
+ set_bit(i, pmu->supported_fixed_pmc_idx);
+ set_bit(INTEL_PMC_MAX_GENERIC + i, pmu->all_valid_pmc_idx);
+ pmu->fixed_ctr_ctrl_mask &= ~(0xbull << (i * 4));
+ counter_mask &= ~BIT_ULL(INTEL_PMC_MAX_GENERIC + i);
+ }
edx.split.bit_width_fixed = min_t(int, edx.split.bit_width_fixed,
kvm_pmu_cap.bit_width_fixed);
pmu->counter_bitmask[KVM_PMC_FIXED] =
@@ -565,10 +586,6 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
setup_fixed_pmc_eventsel(pmu);
}

- for (i = 0; i < pmu->nr_arch_fixed_counters; i++)
- pmu->fixed_ctr_ctrl_mask &= ~(0xbull << (i * 4));
- counter_mask = ~(((1ull << pmu->nr_arch_gp_counters) - 1) |
- (((1ull << pmu->nr_arch_fixed_counters) - 1) << INTEL_PMC_IDX_FIXED));
pmu->global_ctrl_mask = counter_mask;
pmu->global_ovf_ctrl_mask = pmu->global_ctrl_mask
& ~(MSR_CORE_PERF_GLOBAL_OVF_CTRL_OVF_BUF |
@@ -585,11 +602,6 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
pmu->raw_event_mask |= (HSW_IN_TX|HSW_IN_TX_CHECKPOINTED);
}

- bitmap_set(pmu->all_valid_pmc_idx,
- 0, pmu->nr_arch_gp_counters);
- bitmap_set(pmu->all_valid_pmc_idx,
- INTEL_PMC_MAX_GENERIC, pmu->nr_arch_fixed_counters);
-
perf_capabilities = vcpu_get_perf_capabilities(vcpu);
if (cpuid_model_is_consistent(vcpu) &&
(perf_capabilities & PMU_CAP_LBR_FMT))
@@ -605,6 +617,9 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
pmu->pebs_enable_mask = counter_mask;
pmu->reserved_bits &= ~ICL_EVENTSEL_ADAPTIVE;
for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
+ if (!fixed_ctr_is_supported(pmu, i))
+ continue;
+
pmu->fixed_ctr_ctrl_mask &=
~(1ULL << (INTEL_PMC_IDX_FIXED + i * 4));
}

base-commit: d8708b80fa0e6e21bc0c9e7276ad0bccef73b6e7
--
2.40.0