Re: [Xen-devel] [PATCH v5 08/12] x86/hyperv: provide Hyper-V hypercall functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.01.2020 17:15, Wei Liu wrote:
> On Fri, Jan 31, 2020 at 04:35:23PM +0100, Jan Beulich wrote:
>> On 31.01.2020 15:37, Wei Liu wrote:
>>> On Fri, Jan 31, 2020 at 03:24:07PM +0100, Jan Beulich wrote:
>>>> On 29.01.2020 21:20, Wei Liu wrote:
>>>>> I tried using the asm(".equ ..") trick but hit a problem with %c again.
>>>>>
>>>>> mm.c:5736:5: error: invalid 'asm': operand is not a condition code, 
>>>>> invalid operand code 'c'
>>>>>                asm ( ".equ HV_HCALL_PAGE, %c0; .global HV_HCALL_PAGE"
>>>>
>>>> Would you mind also indicating what the input operand actually
>>>> was? According to my looking at gcc sources when you first
>>>> mentioned this (on irc iirc), much depends on it actually be
>>>> recognizable as a constant by the compiler.
>>>
>>> Something along the line:
>>>
>>>   asm ( ".equ HV_HCALL_PAGE, %c0; .global HV_HCALL_PAGE"
>>>        :: "i" (__fix_x_to_virt(FIX_X_HV...))
>>
>> Quite a bit of playing later, %P0 is documented, supported
>> already in gcc 4.1.x, and also used in a few cases by Linux.
>> %p0 would be another documented alternative, but support for
>> this looks to have been introduced later. Not being able to use
>> %c0 here still smells like a bug (and I guess I'll enter one.)
> 
> OK. Let me try that.
> 
> If that turns out successful, do you want me to change the other
> instance to %P0 too?

That was a pretty small value, wasn't it? I guess it might be safer
to switch to %P (and then perhaps also elsewhere in the code base).
But during my playing with it I also noticed there's a signedness
bug (affecting all possible modifiers), so we need to watch out for
results being right in any event.

Jan

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