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SubjectRe: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces
On Mon, Nov 02, 2020 at 02:01:13PM -0600, Segher Boessenkool wrote:
> It just asks the general_operand function, which (for registers) accepts
> the hard registers that are accessible. This does include the float and
> vector etc. registers, normally.
>
> But you usually have a pseudo-register there (which is always allowed
> here), unless you used some register asm variable.

You mean like this:

---
int main(void)
{
register float foo asm ("xmm0") = 0.99f;

asm volatile("movl %0, %%r8d\n\t"
"vmcall\n\t"
:: "g" (foo));

return 0;
}
---

That works ok AFAICT:

---

0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: f3 0f 10 05 00 00 00 movss 0x0(%rip),%xmm0 # c <main+0xc>
b: 00
c: 66 0f 7e c0 movd %xmm0,%eax
10: 41 89 c0 mov %eax,%r8d
13: 0f 01 c1 vmcall
16: b8 00 00 00 00 mov $0x0,%eax
1b: 5d pop %rbp
1c: c3 retq

---

gcc smartypants shuffles it through a GPR before sticking it into %r8.

It works too If I use a float immediate:

---
int main(void)
{
asm volatile("movl %0, %%r8d\n\t"
"vmcall\n\t"
:: "g" (0.99f));

return 0;
}
---

---
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 41 b8 a4 70 7d 3f mov $0x3f7d70a4,%r8d
a: 0f 01 c1 vmcall
d: b8 00 00 00 00 mov $0x0,%eax
12: 5d pop %rbp
13: c3 retq
---

or maybe I'm missing some freaky way to specify the input operand so
that I can make it take a float register. But even if I could, it would
error out when generating the asm, I presume, due to invalid insn or
whatnot.

> And pseudos usually are allocated a simple integer register during
> register allocation, in an asm that is.

Interesting.

> > Might even make people copying from bad examples
> > to go look at the docs first...
>
> Optimism is cool :-)

In my experience so far, documenting stuff better might not always bring
the expected results but sometimes it does move people in the most
unexpected way and miracles happen.

:-)))


--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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