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

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

Avi Kivity wrote:

> If cpu_physical_memory_map() returns NULL, then dma-helpers.c will stop 
> collecting sg entries and submit the I/O.  Tuning that will control how 
> vectored requests are submitted.
> 


 
I understand your suggestion now, something like:

---

diff --git a/dma-helpers.c b/dma-helpers.c
index 96a120c..6c43b97 100644
--- a/dma-helpers.c
+++ b/dma-helpers.c
@@ -96,6 +96,11 @@ static void dma_bdrv_cb(void *opaque, int ret)
     while (dbs->sg_cur_index < dbs->sg->nsg) {
         cur_addr = dbs->sg->sg[dbs->sg_cur_index].base + dbs->sg_cur_byte;
         cur_len = dbs->sg->sg[dbs->sg_cur_index].len - dbs->sg_cur_byte;
+        if (dbs->iov.size + cur_len > DMA_LIMIT) {
+            cur_len = DMA_LIMIT - dbs->iov.size;
+            if (cur_len <= 0)
+                break;
+        }
         mem = cpu_physical_memory_map(cur_addr, &cur_len, !dbs->is_write);
         if (!mem)
             break;

---

would work for me.

However it is difficult to put that code inside cpu_physical_memory_map
since I don't have any reference to link together all the mapping
requests related to the same dma transfer.


> If you problem is specifically with the bdrv_aio_rw_vector bounce 
> buffer, then note that this is a temporary measure until vectored aio is 
> in place, through preadv/pwritev and/or linux-aio IO_CMD_PREADV.  You 
> should either convert to that when it is merged, or implement request 
> splitting in bdrv_aio_rw_vector.
> 
> Can you explain your problem in more detail?



My problem is that my block driver has a size limit for read and write
operations.
When preadv/pwritev are in place I could limit the transfer size
directly in raw_aio_preadv\pwritev but I would also have to update the
iovector size field to reflect that and I think is a little bit ugly.