linux-kernel - Re: [PATCH] kvm: x86 mmu: avoid mmu_page_hash lookup for direct

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Message-ID: <20200623155918.GC23842@linux.intel.com>
Date:   Tue, 23 Jun 2020 08:59:18 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Feiner <pfeiner@...gle.com>
Cc:     Jon Cargille <jcargill@...gle.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>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86 mmu: avoid mmu_page_hash lookup for
 direct_map-only VM

On Mon, Jun 22, 2020 at 11:53:48PM -0700, Sean Christopherson wrote:
> If we do get agressive and zap all children (or if my analysis is wrong),
> and prevent the mixed level insansity, then a simpler approach would be to
> skip the lookup if the MMU is direct.  I.e. no need for the per-VM toggle.
> Direct vs. indirect MMUs are guaranteed to have different roles and so the
> direct MMU's pages can't be reused/shared.

Clarification on the above.  Direct and not-guaranteed-to-be-direct MMUs for
a given VM are guaranteed to have different roles, even for nested NPT vs.
NPT, as nested MMUs will have role.guest_mode=1.