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


From: Philippe Mathieu-Daudé
Subject: Re: [PATCH 5/6] block/nvme: Align block pages queue to host page size
Date: Tue, 5 May 2020 11:48:46 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 5/5/20 10:23 AM, Laurent Vivier wrote:
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 <address@hidden>
---
Cc: Cédric Le Goater <address@hidden>
Cc: David Gibson <address@hidden>
Cc: Laurent Vivier <address@hidden>
---
  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.

nvme_init() seems OK, but you are right, I missed nvme_refresh_limits()... Thanks!


Thanks,
Laurent






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