Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

[Yu Zhang]

On Tue, Feb 09, 2021 at 08:46:42AM +0100, Paolo Bonzini wrote:
> On 09/02/21 04:33, Yu Zhang wrote:
> > On Mon, Feb 08, 2021 at 05:47:22PM +0100, Paolo Bonzini wrote:
> > > On 08/02/21 14:49, Yu Zhang wrote:
> > > > On Mon, Feb 08, 2021 at 12:36:57PM +0100, Paolo Bonzini wrote:
> > > > > On 07/02/21 13:22, Yu Zhang wrote:
> > > > > > In shadow page table, only leaf SPs may be marked as unsync.
> > > > > > And for non-leaf SPs, we use unsync_children to keep the number
> > > > > > of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
> > > > > > shall always be zero for the root SP, , hence no need to check
> > > > > > it. Instead, a warning inside mmu_sync_children() is added, in
> > > > > > case someone incorrectly used it.
> > > > > > 
> > > > > > Also, clarify the mmu_need_write_protect(), by moving the warning
> > > > > > into kvm_unsync_page().
> > > > > > 
> > > > > > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > > > > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > > > 
> > > > > This should really be more of a Co-developed-by, and there are a couple
> > > > > adjustments that could be made in the commit message.  I've queued the patch
> > > > > and I'll fix it up later.
> > > > 
> > > > Indeed. Thanks for the remind, and I'll pay attention in the future. :)
> > > 
> > > Also:
> > > 
> > > arch/x86/kvm/mmu/mmu.c: In function ‘mmu_sync_children’:
> > > arch/x86/kvm/mmu/mmu.c:2002:17: error: ‘sp’ is used uninitialized in this
> > > function [-Werror=uninitialized]
> > >    WARN_ON_ONCE(sp->unsync);
> > 
> > Oops. This is wrong. Should be WARN_ON_ONCE(parent->unsync);
> > 
> > > 
> > > so how was this tested?
> > > 
> > 
> > I ran access test in kvm-unit-test for previous version, which hasn't
> > this code(also in my local repo "enable_ept" was explicitly set to
> > 0 in order to test the shadow mode). But I did not test this one. I'm
> > truely sorry for the negligence - even trying to compile should make
> > this happen!
> > 
> > Should we submit another version? Any suggestions on the test cases?
> 
> Yes, please send v3.
> 
> The commit message can be:
> 
> In shadow page table, only leaf SPs may be marked as unsync; instead, for
> non-leaf SPs, we store the number of unsynced children in unsync_children.
> Therefore, in kvm_mmu_sync_root(), sp->unsync
> shall always be zero for the root SP and there is no need to check
> it.  Remove the check, and add a warning inside mmu_sync_children() to
> assert that the flags are used properly.
> 
> While at it, move the warning from mmu_need_write_protect() to
> kvm_unsync_page().

Thanks Paolo. Will send out v3.

BTW, I just realized that mmu_sync_children() was not triggered by
kvm-unit-test(the access.flat case), so I ran another test by running
a regular VM using shadow, in which I witnessed the synchronization.

B.R.
Yu

> 
> Paolo
>