Re: [PATCH v4 27/32] KVM: SVM: Update svm->ldr_reg cache even if LDR is "bad"

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 08, 2022, Maxim Levitsky wrote:
> On Sat, 2022-10-01 at 00:59 +0000, Sean Christopherson wrote:
> > Update SVM's cache of the LDR even if the new value is "bad".  Leaving
> > stale information in the cache can result in KVM missing updates and/or
> > invalidating the wrong entry, e.g. if avic_invalidate_logical_id_entry()
> > is triggered after a different vCPU has "claimed" the old LDR.
> > 
> > Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/svm/avic.c | 17 +++++++----------
> >  1 file changed, 7 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 2b640c73f447..4b6fc9d64f4d 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -539,23 +539,24 @@ static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
> >  	return &logical_apic_id_table[index];
> >  }
> >  
> > -static int avic_ldr_write(struct kvm_vcpu *vcpu, u8 g_physical_id, u32 ldr)
> > +static void avic_ldr_write(struct kvm_vcpu *vcpu, u8 g_physical_id, u32 ldr)
> >  {
> >  	bool flat;
> >  	u32 *entry, new_entry;
> >  
> > +	if (!ldr)
> > +		return;
> > +
> The avic_get_logical_id_entry already returns NULL in this case, so I don't think
> that this is needed.

Hmm, yeah, it's not strictly needed.  I think I kept it because of the explicit
check that previously existed in avic_handle_ldr_update().  Part of me likes the
explicit check, but avic_invalidate_logical_id_entry() doesn't manually guard
against ldr==0, to I agree it's better to drop this for consistency.