Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

Paolo Bonzini
2024-01-11T10:13:19Z

On Wed, Jan 10, 2024 at 1:23 AM Sean Christopherson <seanjc@google.com> wrote:
> Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query
> whether or not the CPU supports 5-level EPT paging.

I think this is a good idea independent of the selftests issue.

For selftests, we could get similar info from the feature MSR
mechanism, i.e. KVM_GET_MSRS(MSR_IA32_VMX_EPT_VPID_CAP), but only on
Intel and only if nested virtualization is enabled, so that's
inferior.

A better idea for selftests is to add a new KVM_CAP_PHYS_ADDR_SIZE,
which could be implemented by all architectures and especially by both
x86 vendors. However, I am not sure for example if different VM types
(read: TDX?) could have different maximum physical addresses, and that
would have to be taken into consideration when designing the API.

Paolo

> tip-tree folks, this is obviously not technically KVM code, but I'd like to
> take this through the KVM tree so that we can use the information to fix
> KVM selftests (hopefully this cycle).
>
> arch/x86/include/asm/vmxfeatures.h | 1 +
> arch/x86/kernel/cpu/feat_ctl.c | 2 ++
> 2 files changed, 3 insertions(+)
>
> diff --git a/arch/x86/include/asm/vmxfeatures.h b/arch/x86/include/asm/vmxfeatures.h
> index c6a7eed03914..266daf5b5b84 100644
> --- a/arch/x86/include/asm/vmxfeatures.h
> +++ b/arch/x86/include/asm/vmxfeatures.h
> @@ -25,6 +25,7 @@
> #define VMX_FEATURE_EPT_EXECUTE_ONLY ( 0*32+ 17) /* "ept_x_only" EPT entries can be execute only */
> #define VMX_FEATURE_EPT_AD ( 0*32+ 18) /* EPT Accessed/Dirty bits */
> #define VMX_FEATURE_EPT_1GB ( 0*32+ 19) /* 1GB EPT pages */
> +#define VMX_FEATURE_EPT_5LEVEL ( 0*32+ 20) /* 5-level EPT paging */
>
> /* Aggregated APIC features 24-27 */
> #define VMX_FEATURE_FLEXPRIORITY ( 0*32+ 24) /* TPR shadow + virt APIC */
> diff --git a/arch/x86/kernel/cpu/feat_ctl.c b/arch/x86/kernel/cpu/feat_ctl.c
> index 03851240c3e3..1640ae76548f 100644
> --- a/arch/x86/kernel/cpu/feat_ctl.c
> +++ b/arch/x86/kernel/cpu/feat_ctl.c
> @@ -72,6 +72,8 @@ static void init_vmx_capabilities(struct cpuinfo_x86 *c)
> c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_AD);
> if (ept & VMX_EPT_1GB_PAGE_BIT)
> c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_1GB);
> + if (ept & VMX_EPT_PAGE_WALK_5_BIT)
> + c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_5LEVEL);
>
> /* Synthetic APIC features that are aggregates of multiple features. */
> if ((c->vmx_capability[PRIMARY_CTLS] & VMX_F(VIRTUAL_TPR)) &&
>
> base-commit: 1c6d984f523f67ecfad1083bb04c55d91977bb15
> --
> 2.43.0.472.g3155946c3a-goog