Re: [PATCH v2 02/12] KVM: x86: tag the instructions which are used to write page table

Avi Kivity
2011-08-03T09:25:56Z

On 08/03/2011 12:24 PM, Xiao Guangrong wrote:
> > Maybe it's better to emulate if we can't find a fix for that.
> >
> > One way would be to emulate every 20 instructions; this breaks us out of the loop but reduces costly emulations to 5%.
> >
>
> After much thought about this, may be this optimization is not good since:
> - it is little complex
> - this optimization is only applied to the instruction emulation caused by #PF
> - it does not improve too much:
> if we emulate the instruction, we need to do:
> - decode instruction
> - emulate it
> - zap shadow pages
> And do this, it can return to the guest, the guest can run the next instruction
>
> if we retry the instruction, we need to do:
> - decode instruction
> - zap shadow pages
> then return to the guest and retry the instruction, however, we will get page fault
> again(since the mapping is still read-only), so we will get another VM-exit and need
> to do:
> # trigger page fault
> - handle the page fault and change the mapping to writable
> - retry the instruction
> until now, the guest can run the next instruction
>
> So, i do not think the new way is better, your opinion？

We can change to writeable and zap in the same exit, no? Basically call
page_fault() again after zapping.

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