linux-kernel - Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable

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Message-ID: <YlbtEorfabzkRucF@google.com>
Date:   Wed, 13 Apr 2022 15:32:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        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@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level
 expanded pagetable

On Wed, Apr 13, 2022, Paolo Bonzini wrote:
> On 4/13/22 16:42, Sean Christopherson wrote:
> > On Wed, Apr 13, 2022, Paolo Bonzini wrote:
> > > On 4/12/22 23:31, Sean Christopherson wrote:
> > > > We don't need 4 bits for this.  Crossing our fingers that we never had to shadow
> > > > a 2-level guest with a 6-level host, we can do:
> > > > 
> > > > 		unsigned passthrough_delta:2;
> > > > 
> > > Basically, your passthrough_delta is level - glevel in Jiangshan's patches.
> > > You'll need 3 bits anyway when we remove direct later (that would be
> > > passthrough_delta == level).
> > 
> > Are we planning on removing direct?
> 
> I think so, it's redundant and the code almost always checks
> direct||passthrough (which would be passthrough_delta > 0 with your scheme).

It's not redundant, just split out.  E.g. if 3 bits are used for the target_level,
a special value is needed to indicate "direct", otherwise KVM couldn't differentiate
between indirect and direct.  Violent agreement and all that :-)

I'm ok dropping direct and rolling it into target_level, just so long as we add
helpers, e.g. IIUC they would be

static inline bool is_sp_direct(...)
{
	return !sp->role.target_level;
}

static inline bool is_sp_direct_or_passthrough(...)
{
	return sp->role.target_level != sp->role.level;
}

> > > Regarding the naming:
> > > 
> > > * If we keep Jiangshan's logic, I don't like the glevel name very much, any
> > > of mapping_level, target_level or direct_level would be clearer?
> > 
> > I don't love any of these names, especially glevel, because the field doesn't
> > strictly track the guest/mapping/target/direct level.  That could obviously be
> > remedied by making it valid at all times, but then the role would truly need 3
> > bits (on top of direct) to track 5-level guest paging.
> 
> Yes, it would need 3 bits but direct can be removed.
> 
> > > * If we go with yours, I would call the field "passthrough_levels".
> > 
> > Hmm, it's not a raw level though.
> 
> Hence the plural. :)

LOL, I honestly thought that was a typo.  Making it plural sounds like it's passing
through to multiple levels.