Re: [PATCH 4/7] KVM: x86: Add wrapper APIs to reset dirty/available register masks

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 10, 2026, Yosry Ahmed wrote:
> On Tue, Mar 10, 2026 at 5:34 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > Add wrappers for setting regs_{avail,dirty} in anticipation of turning the
> > fields into proper bitmaps, at which point direct writes won't work so
> > well.
> >
> > Deliberately leave the initialization in kvm_arch_vcpu_create() as-is,
> > because the regs_avail logic in particular is special in that it's the one
> > and only place where KVM marks eagerly synchronized registers as available.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/kvm_cache_regs.h | 19 +++++++++++++++++++
> >  arch/x86/kvm/svm/svm.c        |  4 ++--
> >  arch/x86/kvm/vmx/nested.c     |  4 ++--
> >  arch/x86/kvm/vmx/tdx.c        |  2 +-
> >  arch/x86/kvm/vmx/vmx.c        |  4 ++--
> >  5 files changed, 26 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
> > index ac1f9867a234..94e31cf38cb8 100644
> > --- a/arch/x86/kvm/kvm_cache_regs.h
> > +++ b/arch/x86/kvm/kvm_cache_regs.h
> > @@ -105,6 +105,25 @@ static __always_inline bool kvm_register_test_and_mark_available(struct kvm_vcpu
> >         return arch___test_and_set_bit(reg, (unsigned long *)&vcpu->arch.regs_avail);
> >  }
> >
> > +static __always_inline void kvm_reset_available_registers(struct kvm_vcpu *vcpu,
> > +                                                         u32 available_mask)
> 
> Not closely following this series and don't know this code well, but
> this API is very confusing for me tbh. Especially in comparison with
> kvm_reset_dirty_registers().
> 
> Maybe rename this to kvm_clear_available_registers(), and pass in a
> "clear_mask", then reverse the polarity:
> 
> vcpu->arch.regs_avail &= ~clear_mask;

Oh, yeah, I can do something like that.  I originally misread the TDX code and
thought it was explicitly setting regs_avail, and so came up with a roundabout
name.  I didn't revisit the naming or the polarity of the param once I realized
all callers could use the same scheme.

No small part of me is tempted to turn it into a straigh "set" though, unless I'm
missing something, the whole &= business is an implementation quirk.

> Most callers are already passing in an inverse of a mask, so might as
> well pass the mask as-is and invert it here, and it helps make the
> name clear, we're passing in a bitmask to clear from regs_avail.