linux-kernel - Re: [RFC PATCH] x86/arch_prctl: Add ARCH_SET

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Date:   Mon, 18 Jun 2018 13:22:36 -0400
From:   Keno Fischer <keno@...iacomputing.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Andi Kleen <andi@...stfloor.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Kyle Huey <khuey@...ehuey.com>,
        "Robert O'Callahan" <robert@...llahan.org>
Subject: Re: [RFC PATCH] x86/arch_prctl: Add ARCH_SET_XCR0 to mask XCR0 per-thread

On Mon, Jun 18, 2018 at 12:16 PM, Dave Hansen
<dave.hansen@...ux.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.