Re: [PATCH 1/4] KVM: x86: Allow vendor code to disable quirks

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/3/25 02:15, Yan Zhao wrote:
> On Sat, Mar 01, 2025 at 02:34:25AM -0500, Paolo Bonzini wrote:
>> In some cases, the handling of quirks is split between platform-specific
>> code and generic code, or it is done entirely in generic code, but the
>> relevant bug does not trigger on some platforms; for example,
>> KVM_X86_QUIRK_CD_NW_CLEARED is only applicable to AMD systems.  In that
>> case, allow unaffected vendor modules to disable handling of the quirk.
>>
>> The quirk remains available in KVM_CAP_DISABLE_QUIRKS2, because that API
>> tells userspace that KVM *knows* that some of its past behavior was bogus
>> or just undesirable.  In other words, it's plausible for userspace to
>> refuse to run if a quirk is not listed by KVM_CAP_DISABLE_QUIRKS2.
>>
>> In kvm_check_has_quirk(), in addition to checking if a quirk is not
>> explicitly disabled by the user, also verify if the quirk applies to
>> the hardware.
>>
>> 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/vmx/vmx.c |  1 +
>>   arch/x86/kvm/x86.c     |  1 +
>>   arch/x86/kvm/x86.h     | 12 +++++++-----
>>   3 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 486fbdb4365c..75df4caea2f7 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -8506,6 +8506,7 @@ __init int vmx_hardware_setup(void)
>>   
>>   	kvm_set_posted_intr_wakeup_handler(pi_wakeup_handler);
>>   
>> +	kvm_caps.inapplicable_quirks = KVM_X86_QUIRK_CD_NW_CLEARED;
> 
> As you mentioned, KVM_X86_QUIRK_CD_NW_CLEARED has no effect on Intel's
> platforms, no matter kvm_check_has_quirk() returns true or false.
> So, what's the purpose to introduce kvm_caps.inapplicable_quirks?

The purpose is to later mark IGNORE_GUEST_PAT as inapplicable, so that 
the relevant code does not run on AMD.  However you have a point here:

> One concern is that since KVM_X86_QUIRK_CD_NW_CLEARED is not for Intel
> platforms, it's unnatural for Intel's code to add it into the
> kvm_caps.inapplicable_quirks.

So let's instead have kvm-amd.ko clear it from inapplicable_quirks.  And 
likewise kvm-intel.ko can clear IGNORE_GUEST_PAT.

Paolo