Re: [PATCH 2/4] KVM: x86: Introduce supported_quirks to block disabling quirks

[Yan Zhao <yan.y.zhao@intel.com>]

On Sat, Mar 01, 2025 at 02:34:26AM -0500, Paolo Bonzini wrote:
> From: Yan Zhao <yan.y.zhao@intel.com>
> 
> Introduce supported_quirks in kvm_caps to store platform-specific force-enabled
> quirks.  Any quirk removed from kvm_caps.supported_quirks will never be
> included in kvm->arch.disabled_quirks, and will cause the ioctl to fail if
> passed to KVM_ENABLE_CAP(KVM_CAP_DISABLE_QUIRKS2).
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> Message-ID: <20250224070832.31394-1-yan.y.zhao@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 7 ++++---
>  arch/x86/kvm/x86.h | 2 ++
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fd0a44e59314..a97e58916b6a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4782,7 +4782,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		r = enable_pmu ? KVM_CAP_PMU_VALID_MASK : 0;
>  		break;
>  	case KVM_CAP_DISABLE_QUIRKS2:
> -		r = KVM_X86_VALID_QUIRKS;
> +		r = kvm_caps.supported_quirks;
As the concern raised in [1], it's confusing for
KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be present on AMD's platforms while not
present on Intel's non-self-snoop platforms.

What about still returning KVM_X86_VALID_QUIRKS here and only having
kvm_caps.supported_quirks to filter disabled_quirks?

So, for KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT, it's still present when userspace
queries KVM_CAP_DISABLE_QUIRKS2, but it fails when userspace tries to disable
the quirk on Intel platforms without self-snoop.

Not sure if it will cause confusion to the userspace though.

Or what about introduce kvm_caps.force_enabled_quirk?
if (!static_cpu_has(X86_FEATURE_SELFSNOOP))
        kvm_caps.force_enabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;


static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
{
        return !(kvm->arch.disabled_quirks & quirk) |
               (kvm_caps.force_enabled_quirks & quirk);
}

[1] https://lore.kernel.org/all/Z8UBpC76CyxCIRiU@yzhao56-desk.sh.intel.com/
>  		break;
>  	case KVM_CAP_X86_NOTIFY_VMEXIT:
>  		r = kvm_caps.has_notify_vmexit;
> @@ -6521,11 +6521,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  	switch (cap->cap) {
>  	case KVM_CAP_DISABLE_QUIRKS2:
>  		r = -EINVAL;
> -		if (cap->args[0] & ~KVM_X86_VALID_QUIRKS)
> +		if (cap->args[0] & ~kvm_caps.supported_quirks)
>  			break;
>  		fallthrough;
>  	case KVM_CAP_DISABLE_QUIRKS:
> -		kvm->arch.disabled_quirks = cap->args[0];
> +		kvm->arch.disabled_quirks = cap->args[0] & kvm_caps.supported_quirks;
Will this break the uapi of KVM_CAP_DISABLE_QUIRKS?
My understanding is that only KVM_CAP_DISABLE_QUIRKS2 filters out invalid
quirks.

>  		r = 0;
>  		break;
>  	case KVM_CAP_SPLIT_IRQCHIP: {
> @@ -9775,6 +9775,7 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>  		kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
>  		kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
>  	}
> +	kvm_caps.supported_quirks = KVM_X86_VALID_QUIRKS;
>  	kvm_caps.inapplicable_quirks = 0;
>  
>  	rdmsrl_safe(MSR_EFER, &kvm_host.efer);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 9af199c8e5c8..f2672b14388c 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -34,6 +34,8 @@ struct kvm_caps {
>  	u64 supported_xcr0;
>  	u64 supported_xss;
>  	u64 supported_perf_cap;
> +
> +	u64 supported_quirks;
>  	u64 inapplicable_quirks;
>  };
>  
> -- 
> 2.43.5
> 
>