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Date:   Mon, 2 Nov 2020 23:54:39 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     shuo.a.liu@intel.com, linux-kernel@vger.kernel.org, x86@kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Yu Wang <yu1.wang@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Yakui Zhao <yakui.zhao@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Fengwei Yin <fengwei.yin@intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Peter Zijlstra <peterz@infradead.org>,
        Nick Desaulniers <ndesaulniers@google.com>
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