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

[Borislav Petkov]

On Tue, Sep 22, 2020 at 07:42:58PM +0800, shuo.a.liu@xxxxxxxxx wrote:
> From: Shuo Liu <shuo.a.liu@xxxxxxxxx>
> 
> 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.

https://people.kernel.org/tglx/notes-about-netiquette