Re: [PATCH] x86/boot: Do not test if AC and ID eflags are changeable on x86_64

[Uros Bizjak]

On Fri, Mar 7, 2025 at 2:13 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> >> PUSF et al → pushf
> >>
> >> The -l and -q suffixes have been optional for a long time.
> >
> >No, not in this case. Please see the comment:
> >
> >/*
> >* For building the 16-bit code we want to explicitly specify 32-bit
> >* push/pop operations, rather than just saying 'pushf' or 'popf' and
> >* letting the compiler choose.
> >*/
> >
> >We are building 16-bit code here, and we want PUSHFL, the one with
> >operand size prefix 0x66.
> >
> >Please consider the following code:
> >
> >    .code16
> >    pushf
> >    pushfl
> >
> >as -o push.o push.s
> >
> >objdump -dr -Mdata16 push.o
> >
> >0000000000000000 <.text>:
> >  0:   9c                      pushf
> >  1:   66 9c                   pushfl
> >
> >Uros.
> >
>
> *plonk* I should have remembered (.code16gcc is different then .code16 though.) I wrote the damned things after all...

Please note that while "gcc -m16" emits .code16gcc, "clang -m16" emits
.code16, so in the latter case we don't have ‘pushf’, and ‘popf’
instructions default to 32-bit size. So, the only solution is to
decorate pushfl with operand size prefix in this specific case.

Uros.