linux-kernel - Re: [Xen-devel] [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in /proc/cpuinfo

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Date:   Thu, 15 Sep 2016 15:41:13 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Kyle Huey <me@...ehuey.com>, Jan Beulich <JBeulich@...e.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        DaveHansen <dave.hansen@...ux.intel.com>,
        Huang Rui <ray.huang@....com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        "H. PeterAnvin" <hpa@...or.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE 32-BIT AND 64-BIT" <x86@...nel.org>,
        "Robert O'Callahan" <robert@...llahan.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>, Borislav Petkov <bp@...e.de>,
        Len Brown <len.brown@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Juergen Gross <JGross@...e.com>,
        "open list:X86 ARCHITECTURE 32-BIT AND 64-BIT" 
        <linux-kernel@...r.kernel.org>,
        Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>,
        David Vrabel <david.vrabel@...rix.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
Subject: Re: [Xen-devel] [PATCH v2 2/3] x86 Test and expose CPUID faulting
 capabilities in /proc/cpuinfo

On 09/15/2016 03:11 PM, Kyle Huey wrote:
> On Thu, Sep 15, 2016 at 3:25 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>>> On 15.09.16 at 12:05, <david.vrabel@...rix.com> wrote:
>>> On 14/09/16 22:01, Kyle Huey wrote:
>>>> Xen advertises the underlying support for CPUID faulting but not does pass
>>>> through writes to the relevant MSR, nor does it virtualize it, so it does
>>>> not actually work. For now mask off the relevant bit on MSR_PLATFORM_INFO.
>>> Could you clarify in the commit message that it is PV guests that are
>>> affected.
>> What makes you think HVM ones aren't?
> Testing on EC2, HVM guests are affected as well.  Not sure what to do
> about that.

You could clear capability bit in xen_set_cpu_features() but of course
this assumes you will never again read MSR_PLATFORM_INFO.

-boris