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

Marcelo Tosatti
2012-11-27T23:32:46Z

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.