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.
Good mailing practices for 400: avoid top-posting and trim the reply.
