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

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Message-ID: <20210209033319.w6nfb4s567zuly2c@linux.intel.com>
Date:   Tue, 9 Feb 2021 11:33:19 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     seanjc@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org
Subject: Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

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@...ux.intel.com>
> > > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > 
> > > 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?

Thanks
Yu

> Paolo
>