Date:   Mon, 29 Nov 2021 18:23:37 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, isaku.yamahata@intel.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, erdemaktas@google.com,
        Connor Kuehl <ckuehl@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, isaku.yamahata@gmail.com,
        Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [RFC PATCH v3 46/59] KVM: VMX: Move register caching logic to
 common code
Message-ID: <YaUaqTfzSUB2tpkR@google.com>
References: <cover.1637799475.git.isaku.yamahata@intel.com>
 <2f3c1207f66f44fdd2f3eb0809d552f5632e4b41.1637799475.git.isaku.yamahata@intel.com>
 <87mtlshu66.ffs@tglx>
 <620e127f-59d3-ccad-e0f6-39ca9ee7098e@redhat.com>
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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
>