Re: [PATCH 19/27] KVM: x86/mmu: Use page-track notifiers iff there are external users

[Yan Zhao]

On Wed, Aug 09, 2023 at 07:33:45AM -0700, Sean Christopherson wrote:
> On Wed, Aug 09, 2023, Yan Zhao wrote:
> > On Mon, Aug 07, 2023 at 10:19:07AM -0700, Sean Christopherson wrote:
> > > On Mon, Aug 07, 2023, Like Xu wrote:
> > > > On 23/12/2022 8:57 am, Sean Christopherson wrote:
> > > > > +static inline void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa,
> > > > > +					const u8 *new, int bytes)
> > > > > +{
> > > > > +	__kvm_page_track_write(vcpu, gpa, new, bytes);
> > > > > +
> > > > > +	kvm_mmu_track_write(vcpu, gpa, new, bytes);
> > > > > +}
> > > > 
> > > > The kvm_mmu_track_write() is only used for x86, where the incoming parameter
> > > > "u8 *new" has not been required since 0e0fee5c539b ("kvm: mmu: Fix race in
> > > > emulated page table writes"), please help confirm if it's still needed ? Thanks.
> > > > A minor clean up is proposed.
> > > 
> > > Hmm, unless I'm misreading things, KVMGT ultimately doesn't consume @new either.
> > > So I think we can remove @new from kvm_page_track_write() entirely.
> > Sorry for the late reply.
> > Yes, KVMGT does not consume @new and it reads the guest PTE again in the
> > page track write handler.
> > 
> > But I have a couple of questions related to the memtioned commit as
> > below:
> > 
> > (1) If "re-reading the current value of the guest PTE after the MMU lock has
> > been acquired", then should KVMGT also acquire the MMU lock too?
> 
> No.  If applicable, KVMGT should read the new/current value after acquiring
> whatever lock protects the generation (or update) of the shadow entries.  I
> suspect KVMGT already does this, but I don't have time to confirm that at this
I think the mutex lock and unlock of info->vgpu_lock you added in
kvmgt_page_track_write() is the counterpart :)

> exact memory.
> 
> The race that was fixed in KVM was:
> 
>   vCPU0         vCPU1   
>   write X
>                  write Y
>                  sync SPTE w/ Y
>   sync SPTE w/ X
> 
> Reading the value after acquiring mmu_lock ensures that both vCPUs will see whatever
> value "loses" the race, i.e. whatever written value is processed second ('Y' in the
> above sequence).
I suspect that vCPU0 may still generate a wrong SPTE if vCPU1 wrote 4
bytes while vCPU0 wrote 8 bytes, though the chances are very low.


> 
> > If so, could we move the MMU lock and unlock into kvm_page_track_write()
> > as it's common.
> > 
> > (2) Even if KVMGT consumes @new,
> > will kvm_page_track_write() be called for once or twice if there are two
> > concurent emulated write?
> 
> Twice, kvm_page_track_write() is wired up directly to the emulation of the write,
> i.e. there is no batching.