linux-kernel - Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

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Date:   Mon, 5 Feb 2018 20:24:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

On 02/02/2018 21:28, Konrad Rzeszutek Wilk wrote:
>>>> Nothing would *set* the IBPB bit though, since that's a "virtual" bit
>>>> on Intel hardware. The comment explains why we have that |= F(IBPB),
>>>> and if the comment wasn't true, we wouldn't need that code either.
>>> But this seems wrong. That is on Intel CPUs we will advertise on
>>> AMD leaf that the IBPB feature is available.
>>>
>>> Shouldn't we just check to see if the machine is AMD before advertising
>>> this bit?
>> No. The AMD feature bits give us more fine-grained support for exposing
>> IBPB or IBRS alone, so we expose those bits on Intel too.
> But but.. that runs smack against the idea of exposing a platform that
> is as close to emulating the real hardware as possible.
> 
> As in I would never expect an Intel CPU to expose the IBPB on the 0x8000_0008
> leaf. Hence KVM (nor any hypervisor) should not do it either.

This is KVM_GET_*SUPPORTED*_CPUID.  The actual CPUID bits that are
exposed (and also which CPUID leafs are there, even though this one is
present in both Intel and AMD) are determined by userspace.

Paolo