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

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Date:   Thu, 26 May 2022 11:33:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        David Matlack <dmatlack@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <jiangshan.ljs@...group.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>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE
 paging mode

On 5/26/22 10:38, Lai Jiangshan wrote:
>> (Although the APM does say that "modern processors" do not pre-load
>> PDPTEs.)

This changed between the Oct 2020 and Nov 2021, so I suppose the change 
was done in Zen 3.

> Oh, I also missed the fact that L1 is the host when emulating it.
> 
> The code is for host-mode (L1)'s nested_cr3 which is using the
> traditional PAE PDPTEs loading and checking.
> 
> So using caches is the only correct way, right?

The caching behavior for NPT PDPTEs does not matter too much.  What 
matters is that a PDPTE with reserved bits should cause a #NPF at usage 
time rather than a VMentry failure or a #NPF immediately after VMentry.

Paolo