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

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/3/25 02:23, Yan Zhao wrote:
> 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.

To make it less confusing, let's rename it to 
KVM_X86_QUIRK_IGNORE_GUEST_PAT.  So if userspace wants to say "I don't 
want KVM to ignore guest's PAT!", it can do so easily:

- it can check unconditionally that the quirk is included in 
KVM_CAP_DISABLE_QUIRKS2, and it will pass on both Intel with self-snoop 
with AMD;

- it can pass it unconditionally to KVM_X86_QUIRK_IGNORE_GUEST_PAT, 
knowing that PAT will be honored.

But KVM_CHECK_EXTENSION(KVM_CAP_DISABLE_QUIRKS2) will *not* include the 
quirk on Intel without self-snoop, which lets userspace detect the 
condition and raise an error.  This is better than introducing a new 
case in the API "the bit is returned by KVM_CHECK_EXTENSION, but 
rejected by KVM_ENABLE_CAP".  Such a new case would inevitably lead to 
KVM_CAP_DISABLE_QUIRKS3. :)

> 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;

That would also make it harder for userspace to understand what's going on.

> [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.

The difference between KVM_CAP_DISABLE_QUIRKS and 
KVM_CAP_DISABLE_QUIRKS2 is only that invalid values do not cause an 
error; but userspace cannot know what is in kvm->arch.disabled_quirks, 
so KVM can change the value that is stored there.

Paolo


>>   		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
>>
>>
>