Re: [PATCH 2/3] KVM: x86: let reexecute_instruction work for tdp

Gleb Natapov
2012-11-28T14:01:55Z

On Wed, Nov 28, 2012 at 11:15:13AM +0800, Xiao Guangrong wrote:
> On 11/28/2012 07:32 AM, Marcelo Tosatti wrote:
> > On Tue, Nov 27, 2012 at 11:13:11AM +0800, Xiao Guangrong wrote:
> >>>> +static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> >>>> {
> >>>> - gpa_t gpa;
> >>>> + gpa_t gpa = cr2;
> >>>> pfn_t pfn;
> >>>>
> >>>> - if (tdp_enabled)
> >>>> + if (!ACCESS_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> >>>> return false;
> >>>
> >>> How is indirect_shadow_pages protected? Why is ACCESS_ONCE() being used
> >>> to read it?
> >>
> >> Hi Marcelo,
> >>
> >> It is protected by mmu-lock for it only be changed when mmu-lock is hold. And
> >> ACCESS_ONCE is used on read path avoiding magic optimization from compiler.
> >
> > Please switch to mmu_lock protection, there is no reason to have access
> > to this variable locklessly - not performance critical.
> >
> > For example, there is no use of barriers when modifying the variable.
>
> This is not bad, the worst case is, the direct mmu failed to unprotect the shadow
> pages, (meet indirect_shadow_pages = 0, but there has shadow pages being shadowed.),
> after enter to guest, we will go into reexecute_instruction again, then it will
> remove shadow pages.
>
Isn't the same scenario can happen even with mmu lock around
indirect_shadow_pages access?

> But, i do not have strong opinion on it, i respect your idea! :)
>
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--
Gleb.