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From:   Keno Fischer <keno@juliacomputing.com>
Date:   Mon, 18 Jun 2018 11:13:29 -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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>> 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.

>> At least currently, it is my understanding that `xfeatures_mask` only has
>> user features, am I mistaken about that?
>
> We're slowing adding supervisor support.  I think accounting for
> supervisor features is a requirement for any new XSAVE code.

Sure, I don't think this is in any way incompatible with that (though
probably also informs that we want to keep the memory layout the
same if possible).