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

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

On Mon, Mar 03, 2025 at 05:04:40PM +0100, Paolo Bonzini wrote:
> 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:

Or naming it kvm_caps.platform_disabled_quirks?
> 
> > 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.
Sounds good.