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

Shuo A Liu
2020-10-12T08:49:24Z

On Wed 30.Sep'20 at 12:54:08 +0200, Borislav Petkov wrote:
>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?

Yes, it is.

>
>> 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.

Yes. Just a couple of bytes. Number of instructions is same.
sure, i can change to approach 2)
2) use supported constraint as input with a explicit MOV to R8
in beginning of asm

Thanks
shuo