linux-kernel - Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the CPUID instruction.

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Date:   Wed, 14 Sep 2016 12:36:40 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kyle Huey <me@...ehuey.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Ingo Molnar <mingo@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Mateusz Guzik <mguzik@...hat.com>,
        Alex Thorlton <athorlton@....com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ben Segall <bsegall@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Juergen Gross <jgross@...e.com>,
        Linux API <linux-api@...r.kernel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>, Len Brown <len.brown@...el.com>,
        Huang Rui <ray.huang@....com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the
 CPUID instruction.

On Wed, Sep 14, 2016 at 12:28 PM, Andrew Cooper
<andrew.cooper3@...rix.com> wrote:
> On 14/09/2016 20:23, Boris Ostrovsky wrote:
>> On 09/14/2016 02:52 PM, Andy Lutomirski wrote:
>>> On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey <me@...ehuey.com> wrote:
>>>> On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>>>> You should explicitly check that, if the
>>>>> feature is set under Xen PV, then the MSR actually works as
>>>>> advertised.  This may require talking to the Xen folks to make sure
>>>>> you're testing the right configuration.
>>>> This is interesting.  When running under Xen PV the kernel is allowed
>>>> to read the real value of MSR_PLATFORM_INFO and see that CPUID
>>>> faulting is supported.  But as you suggested, writing to
>>>> MSR_MISC_FEATURES_ENABLES doesn't actually enable CPUID faulting, at
>>>> least not in any way that works.
>>>>
>>>> It's not obvious to me how to test this, because when this feature
>>>> works, CPUID only faults in userspace, not in the kernel.  Is there
>>>> existing code somewhere that runs tests like this in userspace?
>>>>
>>> Andrew, Boris: should we expect Xen PV to do anything sensible when we
>>> write to MSR_PLATFORM_INFO to turn on CPUID faulting?  Should the Xen
>>> PV rdmsr hooks or perhaps the hypervisor mask out the feature if it
>>> isn't going to be supported?
>> The hypervisor uses CPUID faulting so we shouldn't advertise this
>> feature to guests.
>
> In the case that the hardware has faulting, or for any HVM guest, the
> extra cost to making the feature available to the guest is a single
> conditional test in the cpuid path.  This is about as close to zero as a
> feature gets.  We really should be offering the feature to guests, and
> have it actually working.  The issue here is that it is leaking when we
> weren't intending to offer it.

As long as Xen can fix this one way or the other in reasonably short
order, I think I'm okay with having Linux incorrectly think it works
on old Xen hypervisors.

--Andy