linux-kernel - Re: [PATCH V3 00/12] KVM: X86/MMU: Use one-off local shadow page for special roots

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Date:   Thu, 26 May 2022 20:27:02 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL VIRTUAL MACHINE FOR MIPS (KVM/mips)" 
        <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>
Subject: Re: [PATCH V3 00/12] KVM: X86/MMU: Use one-off local shadow page for
 special roots

On Thu, May 26, 2022 at 04:49:09PM +0800, Lai Jiangshan wrote:
> On Sat, May 21, 2022 at 9:16 PM Lai Jiangshan <jiangshanlai@...il.com> wrote:
> >
> > From: Lai Jiangshan <jiangshan.ljs@...group.com>
> >
> > Current code uses mmu->pae_root, mmu->pml4_root, and mmu->pml5_root to
> > setup special roots.  The initialization code is complex and the roots
> > are not associated with struct kvm_mmu_page which causes the code more
> > complex.
> >
> > So add new local shadow pages to simplify it.
> >
> > The local shadow pages are associated with struct kvm_mmu_page and
> > VCPU-local.
> >
> > The local shadow pages are created and freed when the roots are
> > changed (or one-off) which can be optimized but not in the patchset
> > since the re-creating is light way (in normal case only the struct
> > kvm_mmu_page needs to be re-allocated and sp->spt doens't, because
> > it is likely to be mmu->pae_root)
> >
> > The patchset also fixes a possible bug described in:
> > https://lore.kernel.org/lkml/20220415103414.86555-1-jiangshanlai@gmail.com/
> > as patch1.
> >
> 
> Ping and please ignore patch1 and patch9. It would not cause any conflict
> without patch1 and patch9 if both are ignored together.
> 
> The fix is wrong (see new discussion in the above link).  So the possible
> correct fix will not have any conflict with this patchset of one-off
> local shadow page.  I don't want to add extra stuff in this patchset
> anymore.

Yeah I agree with splitting this fix out to a separate patchset, and
ordered after this cleanup so it can be done in one patch.

When you get around to it, can you also implement a kvm-unit-test to
demonstrate the bug? It would be good to have a regression test.

> 
> Thanks
> Lai