Re: [Qemu-devel] [PATCH] honor IDE_DMA_BUF_SECTORS

[Stefano Stabellini]

Avi Kivity wrote:

> Stefano Stabellini wrote:
>> I checked the ide driver in the kernel and it assumes that the max
>> sectors is either 256 or 64K depending on lba support, exactly as qemu does.
>>
>>
>> So now my question is: if I want to reduce the maximum dma request size
>> inside qemu, given that I must fill correctly the guest provided sg
>> list, is it OK to use IDE_DMA_BUF_SECTORS in dma_buf_prepare as I have
>> done in my patch?
>>
>> I don't see any other possible solution, but if you have any other
>> suggestion you are welcome to let me know.
>>   
> 
> Look at the DMA API (dma-helpers.c) which already knows how to split 
> large dma requests.  Splitting is controlled by 
> cpu_physical_memory_map() (which I'm guessing is your real limitation), 
> so you might want to look at that.
> 
> The advantage of this approach is that it will apply to scsi and virtio 
> once they are ported to use the DMA API.
> 
> 



Unfortunately that is not really helpful: after the split done by
cpu_physical_memory_map the iovector is converted in a buffer in
bdrv_aio_rw_vector and then the full length of the buffer is passed on
to the bdrv_aio_write\read for the dma operation.

I need a way to set a maximum limit for the total number of sectors in
the dma operation, much like blk_queue_max_phys_segments in the kernel.

This could also be useful to make sure that we don't allocate bounce
buffers bigger than a predetermined limit.