2021-02-03 07:01:07

by Like Xu

Subject: [PATCH] KVM: vmx/pmu: Add VMCS fields check before exposing LBR_FMT

Before KVM exposes guest LBR_FMT perf capabilities, it needs to check
whether VMCS has GUEST_IA32_DEBUGCTL guest status field and vmx switch
support on IA32_DEBUGCTL MSR (including VM_EXIT_SAVE_DEBUG_CONTROLS
and VM_ENTRY_LOAD_DEBUG_CONTROLS). It helps nested LBR enablement.

Signed-off-by: Like Xu <[email protected]>
---
arch/x86/kvm/vmx/capabilities.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index d1d77985e889..ac3af06953a8 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -378,6 +378,12 @@ static inline bool vmx_pt_mode_is_host_guest(void)
return pt_mode == PT_MODE_HOST_GUEST;
}

+static inline bool cpu_has_vmx_lbr(void)
+{
+ return (vmcs_config.vmexit_ctrl & VM_EXIT_SAVE_DEBUG_CONTROLS) &&
+ (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_DEBUG_CONTROLS);
+}
+
static inline u64 vmx_get_perf_capabilities(void)
{
u64 perf_cap = 0;
@@ -385,7 +391,8 @@ static inline u64 vmx_get_perf_capabilities(void)
if (boot_cpu_has(X86_FEATURE_PDCM))
rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);

- perf_cap &= PMU_CAP_LBR_FMT;
+ if (cpu_has_vmx_lbr())
+ perf_cap &= PMU_CAP_LBR_FMT;

/*
* Since counters are virtualized, KVM would support full
--
2.29.2

2021-02-03 09:45:19

by Paolo Bonzini

Subject: Re: [PATCH] KVM: vmx/pmu: Add VMCS fields check before exposing LBR_FMT

On 03/02/21 07:50, Like Xu wrote:
> Before KVM exposes guest LBR_FMT perf capabilities, it needs to check
> whether VMCS has GUEST_IA32_DEBUGCTL guest status field and vmx switch
> support on IA32_DEBUGCTL MSR (including VM_EXIT_SAVE_DEBUG_CONTROLS
> and VM_ENTRY_LOAD_DEBUG_CONTROLS). It helps nested LBR enablement.
>
> Signed-off-by: Like Xu <[email protected]>
> ---
> arch/x86/kvm/vmx/capabilities.h | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index d1d77985e889..ac3af06953a8 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -378,6 +378,12 @@ static inline bool vmx_pt_mode_is_host_guest(void)
> return pt_mode == PT_MODE_HOST_GUEST;
> }
>
> +static inline bool cpu_has_vmx_lbr(void)
> +{
> + return (vmcs_config.vmexit_ctrl & VM_EXIT_SAVE_DEBUG_CONTROLS) &&
> + (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_DEBUG_CONTROLS);
> +}
> +
> static inline u64 vmx_get_perf_capabilities(void)
> {
> u64 perf_cap = 0;
> @@ -385,7 +391,8 @@ static inline u64 vmx_get_perf_capabilities(void)
> if (boot_cpu_has(X86_FEATURE_PDCM))
> rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);
>
> - perf_cap &= PMU_CAP_LBR_FMT;
> + if (cpu_has_vmx_lbr())
> + perf_cap &= PMU_CAP_LBR_FMT;

This is incorrect in the case where cpu_has_vmx_lbr() is false. You
would need something like

u64 perf_cap = 0;
u64 host_perf_cap = 0;

if (boot_cpu_has(X86_FEATURE_PDCM))
rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);

if (cpu_has_vmx_lbr())
perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;

However, KVM won't run without VM_ENTRY_LOAD_DEBUG_CONTROLS and
VM_EXIT_SAVE_DEBUG_CONTROLS (see setup_vmcs_config), so this change is
not needed either.

Paolo