Messages in this thread | | | Date | Mon, 2 Nov 2020 23:54:39 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces |
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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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