Re: [PATCH 5/6] block/nvme: Align block pages queue to host page size

[Laurent Vivier <lvivier@redhat.com>]

On 05/05/2020 10:00, Laurent Vivier wrote:
> On 04/05/2020 11:46, Philippe Mathieu-Daudé wrote:
>> In nvme_create_queue_pair() we create a page list using
>> qemu_blockalign(), then map it with qemu_vfio_dma_map():
>>
>>   q->prp_list_pages = qemu_blockalign0(bs, s->page_size * NVME_QUEUE_SIZE);
>>   r = qemu_vfio_dma_map(s->vfio, q->prp_list_pages,
>>                         s->page_size * NVME_QUEUE_SIZE, ...);
>>
>> With:
>>
>>   s->page_size = MAX(4096, 1 << (12 + ((cap >> 48) & 0xF)));
>>
>> The qemu_vfio_dma_map() documentation says "The caller need
>> to make sure the area is aligned to page size". While we use
>> multiple s->page_size as alignment, it might be not sufficient
>> on some hosts. Use the qemu_real_host_page_size value to be
>> sure the host alignment is respected.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>> Cc: Cédric Le Goater <clg@kaod.org>
>> Cc: David Gibson <david@gibson.dropbear.id.au>
>> Cc: Laurent Vivier <lvivier@redhat.com>
>> ---
>>  block/nvme.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/nvme.c b/block/nvme.c
>> index 7b7c0cc5d6..bde0d28b39 100644
>> --- a/block/nvme.c
>> +++ b/block/nvme.c
>> @@ -627,7 +627,7 @@ static int nvme_init(BlockDriverState *bs, const char *device, int namespace,
>>  
>>      s->page_size = MAX(4096, 1 << (12 + ((cap >> 48) & 0xF)));
>>      s->doorbell_scale = (4 << (((cap >> 32) & 0xF))) / sizeof(uint32_t);
>> -    bs->bl.opt_mem_alignment = s->page_size;
>> +    bs->bl.opt_mem_alignment = MAX(qemu_real_host_page_size, s->page_size);
> 
> Why don't you replace directly the "4096" in s->page_size by
> qemu_real_host_page_size?
> 
> I think this was the purpose of MAX(4096, ...) to align to the host page
> size.

oh, I see, page_size is used for a lot of things than can be broken, and
this is what is done in bdrv_opt_mem_align() for instance, 4096 is the
sector size not the host page size.

BTW, you need the same change as in nvme_init() in
nvme_refresh_limits(), I think.

Thanks,
Laurent