Re: [PATCH v2 5/5] KVM: MMU: retry #PF for softmmu

Avi Kivity
2010-11-15T09:30:45Z

On 11/15/2010 07:25 AM, Xiao Guangrong wrote:
> On 11/14/2010 06:46 PM, Avi Kivity wrote:
> > On 11/12/2010 08:50 AM, Xiao Guangrong wrote:
> >> Retry #PF for softmmu only when the current vcpu has the same
> >> root shadow page as the time when #PF occurs. it means they
> >> have same paging environment
> >>
> >
>
> Hi Avi,
>
> Thanks for your review.
>
> > The process could have been killed and replaced by another using the
> > same cr3.
>
> Yeah, this 'retry' is unnecessary if the process is killed, but this
> case is infrequent, the most case is the process keeps running and try
> to access the fault address later.

The problem is that if we retry in this case, we install an incorrect spte?

> And, we can get few advantages even if the process have been killed,
> since we can fix the page mapping for the other processes which have
> the same CR3, if other process accessed the fault address, the #PF
> can be avoid. (of course we can't speculate other process can access
> the fault address later)
>
> After all, this is a speculate path, i thinks it can work well in most
> case. :-)
>
> > Or we may be running a guest that uses the same cr3 for all
> > processes.
>
> We can allow to retry #PF in the same CR3 even if there are the different
> processes, since these processes have the same page mapping, the later #PF
> can avoid if the page mapping have been fixed.

The guest may have changed page directories or other levels.

> > Or another thread may have mmap()ed something else over the
> > same address.
>
> The mmap virtual address is also visible for other threads since the threads
> have the same page table, so i think this case is the same as above?

Again, don't we install the wrong spte in this case?

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