Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

Marcelo Tosatti
2012-11-29T00:21:32Z

On Thu, Nov 29, 2012 at 06:40:51AM +0800, Xiao Guangrong wrote:
> On 11/29/2012 05:57 AM, Marcelo Tosatti wrote:
> > On Wed, Nov 28, 2012 at 10:59:35PM +0800, Xiao Guangrong wrote:
> >> On 11/28/2012 10:12 PM, Gleb Natapov wrote:
> >>> On Tue, Nov 27, 2012 at 11:30:24AM +0800, Xiao Guangrong wrote:
> >>>> On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:
> >>>>
> >>>>>>
> >>>>>> - return false;
> >>>>>> +again:
> >>>>>> + page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
> >>>>>> +
> >>>>>> + /*
> >>>>>> + * if emulation was due to access to shadowed page table
> >>>>>> + * and it failed try to unshadow page and re-enter the
> >>>>>> + * guest to let CPU execute the instruction.
> >>>>>> + */
> >>>>>> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> >>>>>> + emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
> >>>>>
> >>>>> Can you explain what is the objective here?
> >>>>>
> >>>>
> >>>> Sure. :)
> >>>>
> >>>> The instruction emulation is caused by fault access on cr3. After unprotect
> >>>> the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
> >>>> if it return 1, mmu can not fix the mapping, we should report the error,
> >>>> otherwise it is good to return to guest and let it re-execute the instruction
> >>>> again.
> >>>>
> >>>> page_fault_count is used to avoid the race on other vcpus, since after we
> >>>> unprotect the target page, other cpu can enter page fault path and let the
> >>>> page be write-protected again.
> >>>>
> >>>> This way can help us to detect all the case that mmu can not be fixed.
> >>>>
> >>> Can you write this in a comment above vcpu->arch.mmu.page_fault()?
> >>
> >> Okay, if Marcelo does not object this way. :)
> >
> > I do object, since it is possible to detect precisely the condition by
> > storing which gfns have been cached.
> >
> > Then, Xiao, you need a way to handle large read-only sptes.
>
> Sorry, Marcelo, i am still confused why read-only sptes can not work
> under this patch?
>
> The code after read-only large spte is is:
>
> + if ((level > PT_PAGE_TABLE_LEVEL &&
> + has_wrprotected_page(vcpu->kvm, gfn, level)) ||
> + mmu_need_write_protect(vcpu, gfn, can_unsync)) {
> pgprintk("%s: found shadow page for %llx, marking ro\n",
> __func__, gfn);
> ret = 1;
>
> It return 1, then reexecute_instruction return 0. It is the same as without
> readonly large-spte.

https://lkml.org/lkml/2012/11/17/75

Does unshadowing work with large sptes at reexecute_instruction? That
is, do we nuke any large read-only sptes that might be causing a certain
gfn to be read-only?

That is, following the sequence there, is the large read-only spte
properly destroyed?