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From:   Keno Fischer <keno@juliacomputing.com>
Date:   Mon, 18 Jun 2018 13:22:36 -0400
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Subject: Re: [RFC PATCH] x86/arch_prctl: Add ARCH_SET_XCR0 to mask XCR0 per-thread
To:     Dave Hansen <dave.hansen@linux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@suse.de>,
        Andi Kleen <andi@firstfloor.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Kyle Huey <khuey@kylehuey.com>,
        "Robert O'Callahan" <robert@ocallahan.org>
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On Mon, Jun 18, 2018 at 12:16 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 06/18/2018 08:13 AM, Keno Fischer wrote:
>>>> 4) Catch the fault thrown by xsaves/xrestors in this situation, update
>>>>     XCR0, redo the xsaves/restores, put XCR0 back and continue
>>>>     execution after the faulting instruction.
>>>
>>> I'm worried about the kernel pieces that go digging in the XSAVE data
>>> getting confused more than the hardware getting confused.
>>
>> So you prefer this option? If so, I can try to have a go at implementing it
>> this way and seeing if I run into any trouble.
>
> No, I'm saying that depending on faults is not a viable solution.  We
> are not guaranteed to get faults in all the cases you would need to fix up.
>
> XSAVE*/XRSTOR* are not even *called* in some of those cases.

Ah, my apologies, I was under the mistaken impression that xsaves also
read xcomp_bv to inform the layout, rather than using the RFBM and then
updating the xcomp_bv field. Let me think about this some more and see
what I can come up with.