linux-kernel - Re: [RFC 14/33] KVM: x86: Add VTL to the MMU role

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Message-ID: <CWVCX8ZD8QQZ.2FVZ6DODV8A6T@amazon.com>
Date:   Fri, 10 Nov 2023 18:52:13 +0000
From:   Nicolas Saenz Julienne <nsaenz@...zon.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-hyperv@...r.kernel.org>, <pbonzini@...hat.com>,
        <vkuznets@...hat.com>, <anelkz@...zon.com>, <graf@...zon.com>,
        <dwmw@...zon.co.uk>, <jgowans@...zon.com>, <kys@...rosoft.com>,
        <haiyangz@...rosoft.com>, <decui@...rosoft.com>, <x86@...nel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: [RFC 14/33] KVM: x86: Add VTL to the MMU role

On Wed Nov 8, 2023 at 5:26 PM UTC, Sean Christopherson wrote:
> On Wed, Nov 08, 2023, Nicolas Saenz Julienne wrote:
> > With the upcoming introduction of per-VTL memory protections, make MMU
> > roles VTL aware. This will avoid sharing PTEs between vCPUs that belong
> > to different VTLs, and that have distinct memory access restrictions.
> >
> > Four bits are allocated to store the VTL number in the MMU role, since
> > the TLFS states there is a maximum of 16 levels.
>
> How many does KVM actually allow/support?  Multiplying the number of possible
> roots by 16x is a *major* change.

AFAIK in practice only VTL0/1 are used. Don't know if Microsoft will
come up with more in the future. We could introduce a CAP that expses
the number of supported VTLs to user-space, and leave it as a compile
option.