Re: [PATCH 13/15] KVM: SVM: Add and use svm_register_cache_reset()

[Lai Jiangshan]

On 2021/11/18 23:37, Paolo Bonzini wrote:
> On 11/8/21 13:44, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > It resets all the appropriate bits like vmx.
> >
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/svm/svm.c |  3 +--
> >   arch/x86/kvm/svm/svm.h | 26 ++++++++++++++++++++++++++
> >   2 files changed, 27 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index b7da66935e72..ba9cfddd2875 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -3969,8 +3969,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
> >       svm->vmcb->control.tlb_ctl = TLB_CONTROL_DO_NOTHING;
> >       vmcb_mark_all_clean(svm->vmcb);
> > -
> > -    kvm_register_clear_available(vcpu, VCPU_EXREG_PDPTR);
> > +    svm_register_cache_reset(vcpu);
> >       /*
> >        * We need to handle MC intercepts here before the vcpu has a chance to
> > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > index 0d7bbe548ac3..1cf5d5e2d0cd 100644
> > --- a/arch/x86/kvm/svm/svm.h
> > +++ b/arch/x86/kvm/svm/svm.h
> > @@ -274,6 +274,32 @@ static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
> >           return !test_bit(bit, (unsigned long *)&vmcb->control.clean);
> >   }
> > +static inline void svm_register_cache_reset(struct kvm_vcpu *vcpu)
> > +{
> > +/*
> > + * SVM_REGS_AVAIL_SET - The set of registers that will be updated in cache on
> > + *            demand.  Other registers not listed here are synced to
> > + *            the cache immediately after VM-Exit.
> > + *
> > + * SVM_REGS_DIRTY_SET - The set of registers that might be outdated in
> > + *            architecture. Other registers not listed here are synced
> > + *            to the architecture immediately when modifying.
> > + *
> > + *            Special case: VCPU_EXREG_CR3 should be in this set due
> > + *            to the fact.  But KVM_REQ_LOAD_MMU_PGD is always
> > + *            requested when the cache vcpu->arch.cr3 is changed and
> > + *            svm_load_mmu_pgd() always syncs the new CR3 value into
> > + *            the architecture.  So the dirty information of
> > + *            VCPU_EXREG_CR3 is not used which means VCPU_EXREG_CR3
> > + *            isn't required to be put in this set.
> > + */
> > +#define SVM_REGS_AVAIL_SET    (1 << VCPU_EXREG_PDPTR)
> > +#define SVM_REGS_DIRTY_SET    (0)
> > +
> > +    vcpu->arch.regs_avail &= ~SVM_REGS_AVAIL_SET;
> > +    vcpu->arch.regs_dirty &= ~SVM_REGS_DIRTY_SET;
> > +}
>
> I think touching regs_dirty is confusing here, so I'd go with this:

It makes the code the same as vmx by clearing all the SVM_REGS_DIRTY_SET
bits.  And the compiler will remove this line of code since
SVM_REGS_DIRTY_SET is (0), and regs_dirty is not touched.

Using VMX_REGS_DIRTY_SET and SVM_REGS_DIRTY_SET and making the code
similar is my intent for patch12,13.  If it causes confusing, I would
like to make a second thought.  SVM_REGS_DIRTY_SET does be special
in svm where VCPU_EXREG_CR3 is in it by definition, but it is not
added into SVM_REGS_DIRTY_SET in the patch just for optimization to allow
the compiler optimizes the line of code out.

I'm Ok to not queue patch12,13,14.

>          vcpu->arch.regs_avail &= ~SVM_REGS_LAZY_LOAD_SET;
>
>          /*
>           * SVM does not use vcpu->arch.regs_dirty.  The only register that
>           * might be out of date in the VMCB is CR3, but KVM_REQ_LOAD_MMU_PGD
>           * is always requested when the cache vcpu->arch.cr3 is changed and
>           * svm_load_mmu_pgd() always syncs the new CR3 value into the VMCB.
>           */
>
> (VMX instead needs VCPU_EXREG_CR3 mostly because it does not want to
> update it unconditionally on exit).
>
> Paolo