Good point! Unfortunately the patch is also subtly wrong. We cannot access
the _sav argument while we have unpopped items on the stack. This is because
_sav is a memory argument referencing a local variable (hence is on-stack).
Hence gcc will probably emit a stack-pointer-relative effective address,
which will be incorrect because the stack pointer is different from what gcc
expects.
I'll have a think about how to fix this one.
-- Keir
On 19/10/07 16:43, "David Lively" <dlively@xxxxxxxxxxxxxxx> wrote:
> The x86_emulate code uses the _PRE_EFLAGS macro to setup eflags
> immediately before executing (an emulated version of) the instruction.
> But _PRE_EFLAGS ends in a "andl" instruction, which clobbers the real
> eflags we've just carefully set up. This fix simply leaves the new
> eflags value on the stack until the final "popf" into eflags.
>
> Signed-off-by: David Lively <dlively@xxxxxxxxxxxxxxx>
>
> diff -r 85791ff698bd xen/arch/x86/x86_emulate.c
> --- a/xen/arch/x86/x86_emulate.c Fri Oct 19 11:31:38 2007 -0400
> +++ b/xen/arch/x86/x86_emulate.c Fri Oct 19 11:31:38 2007 -0400
> @@ -300,7 +300,7 @@ struct operand {
>
> /* Before executing instruction: restore necessary bits in EFLAGS. */
> #define _PRE_EFLAGS(_sav, _msk, _tmp) \
> -/* EFLAGS = (_sav & _msk) | (EFLAGS & ~_msk); */\
> +/* push (_sav & _msk) | (EFLAGS & ~_msk); */\
> "push %"_sav"; " \
> "movl %"_msk",%"_LO32 _tmp"; " \
> "andl %"_LO32 _tmp",("_STK"); " \
> @@ -309,11 +309,12 @@ struct operand {
> "andl %"_LO32 _tmp",("_STK"); " \
> "pop %"_tmp"; " \
> "orl %"_LO32 _tmp",("_STK"); " \
> -"popf; " \
> /* _sav &= ~msk; */ \
> "movl %"_msk",%"_LO32 _tmp"; " \
> "notl %"_LO32 _tmp"; " \
> -"andl %"_LO32 _tmp",%"_sav"; "
> +"andl %"_LO32 _tmp",%"_sav"; " \
> +/* pop EFLAGS */ \
> +"popf; "
>
> /* After executing instruction: write-back necessary bits in EFLAGS. */
> #define _POST_EFLAGS(_sav, _msk, _tmp) \
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel
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