linux-kernel - Re: [PATCH v3 1/3] x86/fpu: Make XCR0 accessors immune to unwanted compiler reordering

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Date:   Fri, 1 Apr 2022 15:14:06 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <linux-pm@...r.kernel.org>
CC:     <tglx@...utronix.de>, <dave.hansen@...ux.intel.com>,
        <peterz@...radead.org>, <bp@...en8.de>, <rafael@...nel.org>,
        <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v3 1/3] x86/fpu: Make XCR0 accessors immune to unwanted
 compiler reordering

On 4/1/2022 11:16 AM, Dave Hansen wrote:
> On 4/1/22 10:58, Dave Hansen wrote:
>> On 3/24/22 19:22, Chang S. Bae wrote:
>>> Some old GCC versions (4.9.x and 5.x) have an issue that incorrectly
>>> reordering volatile asm statements with each other [1]. While this bug was
>>> fixed on later versions (8.1, 7.3, and 6.5), and the kernel's current XCR0
>>> read/write do not appear to be impacted, it is preventive to ensure them on
>>> the program order.
>> I thought you *actually* saw xgetbv() be reordered, though.  Was that on
>> a buggy compiler?

No, sorry, my earlier analysis was naive. The #UD was raised on the 
non-XGETBV1 system because Objtool didn't process 
fpu_state_size_dynamic() at build time. It was only when the TILERELEASE 
opcode was not given. Here is a bit more detail:
https://lore.kernel.org/lkml/aa3ff0f4-d74c-2c84-37c0-0880cabc0dd4@intel.com/

> 
> Actually, reading the gcc bug[1] a bit more, it says:
> 
>> However, correctness here depends on the compiler honouring the
>> ordering of volatile asm statements with respect to other volatile
>> code, and this patch fixes that.
> In your case, there was presumably no volatile code, just a
> fpu_state_size_dynamic() call.  The compiler thought the xgetbv() was
> safe to reorder, and did so.
> 
> So, I'm not quite sure how that bug is relevant.  It just dealt with
> multiple "asm volatile" statements that don't have memory clobbers.  We
> only have one "asm volatile" in play.  Adding the memory clobber should
> make that bug totally irrelevant.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

Yeah, now this patch looks to have created more confusion than fixing 
any real problem. Let me drop this on v4.

Thanks,
Chang