Re: [PATCH v10 071/108] KVM: x86: Allow to update cached values in kvm_user_return_msrs w/o wrmsr

Isaku Yamahata
2022-11-17T20:10:14Z

On Mon, Nov 14, 2022 at 03:36:27PM +0800,
Binbin Wu <binbin.wu@linux.intel.com> wrote:

> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 65541bfebb37..4d4b71c4cdb1 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -440,6 +440,15 @@ void kvm_user_return_msr_init_cpu(void)
> > }
> > EXPORT_SYMBOL_GPL(kvm_user_return_msr_init_cpu);
> > +static void kvm_user_return_register_notifier(struct kvm_user_return_msrs *msrs)
> > +{
> > + if (!msrs->registered) {
> > + msrs->urn.on_user_return = kvm_on_user_return;
> > + user_return_notifier_register(&msrs->urn);
> > + msrs->registered = true;
> > + }
> > +}
> > +
> > int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
> > {
> > struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
> > @@ -455,15 +464,22 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
> > return 1;
> > msrs->values[slot].curr = value;
> > - if (!msrs->registered) {
> > - msrs->urn.on_user_return = kvm_on_user_return;
> > - user_return_notifier_register(&msrs->urn);
> > - msrs->registered = true;
> > - }
> > + kvm_user_return_register_notifier(msrs);
> > return 0;
> > }
> > EXPORT_SYMBOL_GPL(kvm_set_user_return_msr);
> > +/* Update the cache, "curr", and register the notifier */
> > +void kvm_user_return_update_cache(unsigned int slot, u64 value)
> > +{
> > + struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
> > +
> > + WARN_ON_ONCE(!msrs->initialized);
> > + msrs->values[slot].curr = value;
>
> If the value is the same as the cached one, can it just skip notifier
> registration?

We know tha the the user of the function (in the next patch) passes the
different value, the initial reset value in the return of TDH.VP.ENTER. So
such check isn't added.
--
Isaku Yamahata <isaku.yamahata@gmail.com>