Re: [PATCH 09/11] KVM: MMU: simplify mmu_need_write_protect

[Xiao Guangrong]

On 12/15/2015 04:43 PM, Kai Huang wrote:
> On 12/01/2015 02:26 AM, Xiao Guangrong wrote:
> > Now, all non-leaf shadow page are page tracked, if gfn is not tracked
> > there is no non-leaf shadow page of gfn is existed, we can directly
> > make the shadow page of gfn to unsync
> >
> > Signed-off-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mmu.c | 26 ++++++++------------------
> >   1 file changed, 8 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 5a2ca73..f89e77f 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -2461,41 +2461,31 @@ static void __kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> >       kvm_mmu_mark_parents_unsync(sp);
> >   }
> > -static void kvm_unsync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
> > +static bool kvm_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn,
> > +                 bool can_unsync)
> >   {
> >       struct kvm_mmu_page *s;
> >       for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn) {
> > +        if (!can_unsync)
> > +            return true;
> How about moving this right before for_each_gfn_indirect_valid_sp? As can_unsync is passed as
> parameter, so there's no point checking it several times.

We can not do this. What we are doing here is checking if we have shadow page mapping
for 'gfn':
a) if no, it can be writable.
b) if yes, check 'can_unsync' to see if these shadow pages can make to be 'unsync'.

Your suggestion can break the point a).

> A further thinking is can we move it to mmu_need_write_protect? Passing can_unsync as parameter to
> kvm_unsync_pages sounds a little bit odd.
>
> > +
> >           if (s->unsync)
> >               continue;
> >           WARN_ON(s->role.level != PT_PAGE_TABLE_LEVEL);
> How about large page mapping? Such as if guest uses 2M mapping and its shadow is indirect, does
> above WARN_ON still meet? As you removed the PT level check in mmu_need_write_protect.

The lager mapping are on the non-leaf shadow pages which can be figured out by
kvm_page_track_check_mode() before we call this function.

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