Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info

[Sean Christopherson]

On Fri, May 15, 2020 at 04:33:52PM -0400, Vivek Goyal wrote:
> On Fri, May 15, 2020 at 09:18:07PM +0200, Paolo Bonzini wrote:
> > On 15/05/20 20:46, Sean Christopherson wrote:
> > >> The new one using #VE is not coming very soon (we need to emulate it for
> > >> <Broadwell and AMD processors, so it's not entirely trivial) so we are
> > >> going to keep "page not ready" delivery using #PF for some time or even
> > >> forever.  However, page ready notification as #PF is going away for good.
> > > 
> > > And isn't hardware based EPT Violation #VE going to require a completely
> > > different protocol than what is implemented today?  For hardware based #VE,
> > > KVM won't intercept the fault, i.e. the guest will need to make an explicit
> > > hypercall to request the page.
> > 
> > Yes, but it's a fairly simple hypercall to implement.
> > 
> > >> That said, type1/type2 is quite bad. :)  Let's change that to page not
> > >> present / page ready.
> > > 
> > > Why even bother using 'struct kvm_vcpu_pv_apf_data' for the #PF case?  VMX
> > > only requires error_code[31:16]==0 and SVM doesn't vet it at all, i.e. we
> > > can (ab)use the error code to indicate an async #PF by setting it to an
> > > impossible value, e.g. 0xaaaa (a is for async!).  That partciular error code
> > > is even enforced by the SDM, which states:
> > 
> > Possibly, but it's water under the bridge now.
> > And the #PF mechanism also has the problem with NMIs that happen before
> > the error code is read
> > and page faults happening in the handler (you may connect some dots now).
> 
> I understood that following was racy.
> 
> do_async_page_fault <--- kvm injected async page fault
>   NMI happens (Before kvm_read_and_reset_pf_reason() is done)
>    ->do_async_page_fault() (This is regular page fault but it will read
>    			    reason from shared area and will treat itself
> 			    as async page fault)
> 
> So this is racy.
> 
> But if we get rid of the notion of reading from shared region in page
> fault handler, will we not get rid of this race.
> 
> I am assuming that error_code is not racy as it is pushed on stack.
> What am I missing.

Nothing, AFAICT.  As I mentioned in a different mail, CR2 can be squished,
but I don't see how error code can be lost.

But, because CR2 can be squished, there still needs to be an in-memory busy
flag even if error code is used as the host #PF indicator, otherwise the
guest could lose one of the tokens.