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Subject: Re: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in /proc/cpuinfo
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Thu, 15 Sep 2016 13:38:00 -0700
To: Andy Lutomirski <luto@amacapital.net>, Kyle Huey <me@kylehuey.com>
CC: Dave Hansen <dave.hansen@linux.intel.com>,
        "Robert O'Callahan" <robert@ocallahan.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>,
        Juergen Gross <jgross@suse.com>, Borislav Petkov <bp@suse.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Huang Rui <ray.huang@amd.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@lists.xenproject.org>
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On September 14, 2016 6:17:51 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Wed, Sep 14, 2016 at 3:03 PM, Kyle Huey <me@kylehuey.com> wrote:
>> On Wed, Sep 14, 2016 at 2:35 PM, Dave Hansen
>> <dave.hansen@linux.intel.com> wrote:
>>> On 09/14/2016 02:01 PM, Kyle Huey wrote:
>
>>> Is any of this useful to optimize away at compile-time?  We have
>config
>>> options for when we're running as a guest, and this seems like a
>feature
>>> that isn't available when running on bare metal.
>>
>> On the contrary, this is only available when we're on bare metal.
>> Neither Xen nor KVM virtualize CPUID faulting (although KVM correctly
>> suppresses MSR_PLATFORM_INFO's report of support for it).
>
>KVM could easily support this.  If rr starts using it, I think KVM
>*should* add support, possibly even for older CPUs that don't support
>the feature in hardware.
>
>It's too bad that x86 doesn't give us the instruction bytes on a
>fault.  Otherwise we could lazily switch this feature.
>
>--Andy

You can "always" examine the instruction bytes in memory... have to make sure you properly consider the impact of race conditions though.
-- 
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