Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v6 2/5] KVM: x86: Add IBPB support
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        David Woodhouse <dwmw2@infradead.org>
Cc:     KarimAllah Ahmed <karahmed@amazon.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>
References: <1517522386-18410-1-git-send-email-karahmed@amazon.de>
 <1517522386-18410-3-git-send-email-karahmed@amazon.de>
 <20180202174932.GR28192@char.us.oracle.com>
 <1517594544.31953.62.camel@infradead.org>
 <20180202195601.GD28192@char.us.oracle.com>
 <1517602575.31953.74.camel@infradead.org>
 <20180202202857.GI28192@char.us.oracle.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5fae597b-c70e-1610-165e-5afc3ec996a9@redhat.com>
Date:   Mon, 5 Feb 2018 20:24:01 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.5.2
MIME-Version: 1.0
In-Reply-To: <20180202202857.GI28192@char.us.oracle.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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