linux-kernel - Re: [RFC PATCH v3 46/59] KVM: VMX: Move register caching logic to common code

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Date:   Mon, 29 Nov 2021 18:23:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, isaku.yamahata@...el.com,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC PATCH v3 46/59] KVM: VMX: Move register caching logic to
 common code

On Thu, Nov 25, 2021, Paolo Bonzini wrote:
> On 11/25/21 21:11, Thomas Gleixner wrote:
> > > 
> > > Use kvm_x86_ops.cache_reg() in ept_update_paging_mode_cr0() rather than
> > > trying to expose vt_cache_reg() to vmx.c, even though it means taking a
> > > retpoline.  The code runs if and only if EPT is enabled but unrestricted
> > > guest.
> > This sentence does not parse because it's not a proper sentence.

Heh, supposed to be "... but unrestricted guest is disabled".

> > > Only one generation of CPU, Nehalem, supports EPT but not
> > > unrestricted guest, and disabling unrestricted guest without also
> > > disabling EPT is, to put it bluntly, dumb.
> > This one is only significantly better and lacks an explanation what this
> > means for the dumb case.
> 
> Well, it means a retpoline (see paragraph before).

No, the point being made is that, on a CPU that supports Unrestricted Guest (UG),
disabling UG without disabling EPT is really, really stupid.  UG requires EPT, so
disabling EPT _and_ UG is reasonable as there are scenarios where using shadow
paging is desirable.  But inentionally disabling UG and enabling EPT makes no
sense.  It forces KVM to emulate non-trivial amounts of guest code and has zero
benefits for anything other than testing KVM itself.

> why it one wouldn't create a vt.h header with all vt_* functions.
> 
> Paolo
>