Re: [Qemu-devel] [PULL for-2.5 2/4] block: Don't wait serialising for non-COR read requests

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 12/08/2015 02:58 PM, Christian Borntraeger wrote:
[...9
>>>
>>> Not a compiler bug. gcc uses a floating point register 8 to spill
>>> the pointer of blk (which is call saved) submit_request will later
>>> on call  qemu_coroutine_enter and after returning from 
>>> qemu_coroutine_enter, the fpr8 contains junk. Not sure yet, what happened.
>>
>> Coroutines don't save the FPU state, so you're not supposed to use
>> floating point operations inside coroutines. That the compiler spills
>> some integer value into a floating point register is a bit nasty...
> 
> Just checked.  bdrv_aligned_preadv does also use fprs (also for filling
> and spilling). Some versions of gcc seem to like that as the LDGR and LGDR
> instructions are pretty cheap and move the content from/to fprs in a bitwise
> fashion. So this coroutine DOES trash floating point registers.
> 
> Without the patch gcc seems to be fine with the 16 gprs and does not
> spilling/filling from/to fprs in bdrv_aligned_preadv.
> 
> Christian

Kevin,

I am wondering. gcc saves/restores f8 in the generated code for the
coroutine and setjmp/longjmp also save/restore the fprs. why do 
coroutines do not save the FPU state (which code does a light weight
switching)

Christian