linux-kernel - Re: [PATCH 2/2] KVM: X86/MMU: Introduce role.passthrough for shadowing 5-level NPT for 4-level NPT L1 guest

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Date:   Thu, 21 Apr 2022 08:23:32 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: X86/MMU: Introduce role.passthrough for
 shadowing 5-level NPT for 4-level NPT L1 guest

On Thu, Apr 21, 2022 at 1:15 AM Paolo Bonzini <pbonzini@...hat.com> wrote:

>
> Looks good, and is easy to backport; thanks.
>
> However, once we have also your series "KVM: X86/MMU: Use one-off
> special shadow page for special roots", perhaps the passthrough sp can
> be converted to a special root (but now with a shadow_page) as well?
>

It will be much more complicated.

The current code allows fast_pgd_switch() for shadowing 5-level NPT
for 4-level NPT L1 guests because both the host and the guest are
64 bit. Using a special shadow page will disallow fast_pgd_switch().

And it will make using_special_root_page() more complicated.

It will require a different kind of special shadow page to make it
able to do fast switch.