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

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Message-ID: <39b21ede-5d53-0545-631e-165df9ecb7f5@redhat.com>
Date:   Mon, 29 Nov 2021 19:28:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 11/29/21 19:23, Sean Christopherson wrote:
>>>> 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.

Yes, I understand that.

Thomas was asking what it means to "Move register caching logic to 
common code", i.e. what the consequences are.  The missing words at the 
end of the first paragraph didn't make the connection obvious between 
the extra retpoline and the "dumb case".

Paolo