linux-kernel - Re: [PATCH 1/12] KVM: X86: Fix when shadow_root_level=5 && guest root

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Date:   Thu, 9 Dec 2021 01:21:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 1/12] KVM: X86: Fix when shadow_root_level=5 && guest
 root_level<4

On Thu, Dec 09, 2021, Sean Christopherson wrote:
> On Wed, Nov 24, 2021, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@...ux.alibaba.com>
> > 
> > If the is an L1 with nNPT in 32bit, the shadow walk starts with
> > pae_root.
> > 
> > Fixes: a717a780fc4e ("KVM: x86/mmu: Support shadowing NPT when 5-level paging is enabled in host)
> 
> Have you actually run with 5-level nNPT?  I don't have access to hardware, at least
> not that I know of :-)
> 
> I'm staring at kvm_mmu_sync_roots() and don't see how it can possibly work for
> 5-level nNPT with a 4-level NPT guest.

Oh, and fast_pgd_switch() will also break kvm_mmu_sync_prev_roots() / is_unsync_root()
by putting a root into the prev_roots array that doesn't have a shadow page associated
with the root.