On 06/12/16 14:14, Jan Beulich wrote:
> The memory clobber is rather harsh here.
Does removing it actually make a difference? I can't spot anything
which could reasonably be reordered around the asm() blocks.
> However, fic.exn_raised may be
> modified as a side effect, so we need to let the compiler know that all
> of "fic" may be changed (preventing it from moving around accesses to
> the exn_raised field).
>
> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -784,7 +784,7 @@ do {
> })
>
> struct fpu_insn_ctxt {
> - uint8_t insn_bytes;
> + uint8_t insn_bytes; /* Must be first! */
And one single byte. The compiler would previously have caught an
accidental breaking of this requirement.
As an alternative, how about using ACCESS_ONCE() to read exn_raised? It
would allow you to drop the memory clobber and also not generalise the
fic.insn_bytes memory parameter to fic.
~Andrew
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