Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

Borislav Petkov
2020-09-30T10:54:24Z

On Tue, Sep 22, 2020 at 07:42:58PM +0800, shuo.a.liu@intel.com wrote:
> From: Shuo Liu <shuo.a.liu@intel.com>
>
> The Service VM communicates with the hypervisor via conventional
> hypercalls. VMCALL instruction is used to make the hypercalls.
>
> ACRN hypercall ABI:
> * Hypercall number is in R8 register.
> * Up to 2 parameters are in RDI and RSI registers.
> * Return value is in RAX register.

I'm assuming this is already cast in stone in the HV and it cannot be
changed?

> Introduce the ACRN hypercall interfaces. Because GCC doesn't support R8
> register as direct register constraints, here are two ways to use R8 in
> extended asm:
> 1) use explicit register variable as input
> 2) use supported constraint as input with a explicit MOV to R8 in
> beginning of asm
>
> The number of instructions of above two ways are same.
> Asm code from 1)
> 38: 41 b8 00 00 00 80 mov $0x80000000,%r8d
> 3e: 48 89 c7 mov %rax,%rdi
> 41: 0f 01 c1 vmcall
> Here, writes to the lower dword (%r8d) clear the upper dword of %r8 when
> the CPU is in 64-bit mode.
>
> Asm code from 2)
> 38: 48 89 c7 mov %rax,%rdi
> 3b: 49 b8 00 00 00 80 00 movabs $0x80000000,%r8
> 42: 00 00 00
> 45: 0f 01 c1 vmcall
>
> Choose 1) for code simplicity and a little bit of code size
> optimization.

What?

How much "optimization" is this actually? A couple of bytes?

And all that for this

/* Nothing can come between the r8 assignment and the asm: */

restriction?

If it is only a couple of bytes, just do the explicit MOV to %r8 and
f'get about it.

--
Regards/Gruss,
Boris.