linux-kernel - Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE paging mode

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Date:   Fri, 13 May 2022 16:54:21 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Lai Jiangshan <jiangshan.ljs@...group.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Avi Kivity <avi@...hat.com>,
        "open list:KERNEL VIRTUAL MACHINE FOR MIPS (KVM/mips)" 
        <kvm@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE
 paging mode

On Fri, Apr 15, 2022 at 6:33 PM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
>
> When NPT enabled L1 is PAE paging, vcpu->arch.mmu->get_pdptrs() which
> is nested_svm_get_tdp_pdptr() reads the guest NPT's PDPTE from memroy
> unconditionally for each call.
>
> The guest PAE root page is not write-protected.
>
> The mmu->get_pdptrs() in FNAME(walk_addr_generic) might get different
> values every time or it is different from the return value of
> mmu->get_pdptrs() in mmu_alloc_shadow_roots().
>
> And it will cause FNAME(fetch) installs the spte in a wrong sp
> or links a sp to a wrong parent since FNAME(gpte_changed) can't
> check these kind of changes.
>
> Cache the PDPTEs and the problem is resolved.  The guest is responsible
> to info the host if its PAE root page is updated which will cause
> nested vmexit and the host updates the cache when next nested run.
>
> The commit e4e517b4be01 ("KVM: MMU: Do not unconditionally read PDPTE
> from guest memory") fixs the same problem for non-nested case.
>
> Fixes: e4e517b4be01 ("KVM: MMU: Do not unconditionally read PDPTE from guest memory")
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>

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