Re: [RFC PATCH v5 4/4] x86/acrn: Add hypercall for ACRN guest

[Borislav Petkov]

On Thu, Apr 25, 2019 at 06:16:02PM +0800, Zhao, Yakui wrote:
> The parameter register for the VMCALL is predefined in ACRN hypervisor. Now
> the R8 is used to pass the hcall_id.
> It seems that there is no special constraint for R8~R15.
> So the explicit register variable is used so that the R8 can be passed.

If you're going to use the constraint "D" for param1, you can just as
well do

	"=a" (result)

everywhere since you have the letter constraint for %rax instead of
declaring it with "register".

Also, you can completely get rid of those "register" declarations
and let gcc have all the freedom to pass in hcall_id and the other
parameters:

	unsigned long result;

        asm volatile("mov %[hcall_id], %%r8\n\t"
                     "vmcall\n\t"
                     : "=a" (result)
                     : [hcall_id] "g" (hcall_id)
                     : "r8");

        return result;

and %r8 will be in the clobber list so gcc will reload it if needed.

gcc turns it into

0000000000001040 <main>:
    1040:       4c 8b 05 e1 2f 00 00    mov    0x2fe1(%rip),%r8        # 4028 <hcall_id>
    1047:       0f 01 c1                vmcall
    104a:       c3                      retq 
    104b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)

here.

-- 
Regards/Gruss,
    Boris.

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