linux-kernel - Re: [RFC PATCH v2 41/69] KVM: x86: Add infrastructure for stolen GPA bits

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Message-ID: <YQw07f/I72/b58pR@google.com>
Date:   Thu, 5 Aug 2021 18:58:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "erdemaktas@...gle.com" <erdemaktas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ckuehl@...hat.com" <ckuehl@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [RFC PATCH v2 41/69] KVM: x86: Add infrastructure for stolen GPA
 bits

On Thu, Aug 05, 2021, Edgecombe, Rick P wrote:
> On Thu, 2021-08-05 at 17:39 +0000, Sean Christopherson wrote:
> > If we really want to reduce the memory footprint for the common case (TDP
> > MMU), the crud that's used only by indirect shadow pages could be shoved
> > into a different struct by abusing the struct layout and and wrapping
> > accesses to the indirect-only fields with casts/container_of and helpers,
> > e.g.
> > 
> Wow, didn't realize classic MMU was that relegated already. Mostly an
> onlooker here, but does TDX actually need classic MMU support? Nice to
> have?

Gah, bad verbiage on my part.  I didn't me _the_ TDP MMU, I meant "MMU that uses
TDP".  The "TDP MMU" is being enabled by default in upstream, whether or not TDX
needs to support the classic/legacy MMU is an unanswered question.  From a
maintenance perspective, I'd love to say no, but from a "what does Google actually
want to use for TDX" perspective, I don't have a defininitive answer yet :-/

> > The role is already factored into the collision logic.
>
> I mean how aliases of the same gfn don't necessarily collide and the
> collisions counter is only incremented if the gfn/stolen matches, but
> not if the role is different.

Ah.  I still think it would Just Work.  The collisions tracking is purely for
stats, presumably used in the past to see if the hash size was effective.  If
a hash lookup collides then it should be accounted, _why_ it collided doesn't
factor in to the stats.

Your comments about the hash behavior being different still stand, though for
TDX they wouldn't matter in practice since KVM is hosed if it has both shared and
private versions of a shadow page.