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

[Chang S. Bae]

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@xxxxxxxxx/

> 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