Re: [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

[Sean Christopherson]

On Thu, Jul 15, 2021, Dave Hansen wrote:
> On 7/15/21 11:56 AM, Sean Christopherson wrote:
> >>>> +       /* Retry if another processor is modifying the RMP entry. */
> >>>> +       do {
> >>>> +               /* Binutils version 2.36 supports the PSMASH mnemonic. */
> >>>> +               asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> >>>> +                             : "=a"(ret)
> >>>> +                             : "a"(spa)
> >>>> +                             : "memory", "cc");
> >>>> +       } while (ret == FAIL_INUSE);
> >>> Should there be some retry limit here for safety? Or do we know that
> >>> we'll never be stuck in this loop? Ditto for the loop in rmpupdate.
> >> It's probably fine to just leave this.  While you could *theoretically*
> >> lose this race forever, it's unlikely to happen in practice.  If it
> >> does, you'll get an easy-to-understand softlockup backtrace which should
> >> point here pretty quickly.
> > But should failure here even be tolerated?  The TDX cases spin on flows that are
> > _not_ due to (direct) contenion, e.g. a pending interrupt while flushing the
> > cache or lack of randomness when generating a key.  In this case, there are two
> > CPUs racing to modify the RMP entry, which implies that the final state of the
> > RMP entry is not deterministic.
> 
> I was envisioning that two different CPUs could try to smash two
> *different* 4k physical pages, but collide since they share
> a 2M page.
> 
> But, in patch 33, this is called via:
> 
> > +		write_lock(&kvm->mmu_lock);
> > +
> > +		switch (op) {
> > +		case SNP_PAGE_STATE_SHARED:
> > +			rc = snp_make_page_shared(vcpu, gpa, pfn, level);
> ...
> 
> Which should make collisions impossible.  Did I miss another call-site?

Ya, there's more, e.g. sev_snp_write_page_begin() and snp_handle_rmp_page_fault(),
both of which run without holding mmu_lock.  The PSMASH operation isn't too
concerning, but the associated RMPUDATE is most definitely a concern, e.g. if two
vCPUs are trying to access different variants of a page.  It's ok if KVM's
"response" in such a situation does weird things to the guest, but one of the
two operations should "win", which I don't think is guaranteed if multiple RMP
violations are racing.

I'll circle back to this patch after I've gone through the KVM MMU changes.