Re: [PATCH] guestcopy: evaluate {,__}copy{,_field}_to_guest*() arguments just once

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.04.2020 23:28, Julien Grall wrote:
> On 01/04/2020 15:29, Jan Beulich wrote:
>> There's nothing wrong with having e.g.
>>
>>      copy_to_guest(uarg, ptr++, 1);
>>
>> yet until now this would increment "ptr" twice.
> 
> Is there such use in Xen today? I guess not as we would have noticed any 
> issue.

I'm not aware of any.

>> --- a/xen/include/asm-arm/guest_access.h
>> +++ b/xen/include/asm-arm/guest_access.h
>> @@ -79,7 +79,7 @@ int access_guest_memory_by_ipa(struct do
>>       const typeof(*(ptr)) *_s = (ptr);                   \
>>       char (*_d)[sizeof(*_s)] = (void *)(hnd).p;          \
>>       void *__maybe_unused _t = (hnd).p;                  \
>> -    ((void)((hnd).p == (ptr)));                         \
>> +    (void)((hnd).p == _s);                              \
> 
> May I ask why this is a problem with 'ptr' but not 'hnd'?
> Wouldn't it theorically possible to have an array of handle?

Theoretically yes, but I view issues with the handle as far less
likely than issues with any of the other parameters (in particular
I don't see what an array of handles could be used for). So yes,
if we want to be eager, we could deal with that one as well.

Jan