Re: [PATCH] block: use the request length for iov alignment

[Keith Busch]

On Tue, Sep 13, 2022 at 03:20:23PM +0100, Damien Le Moal wrote:
> On 2022/09/13 15:12, Keith Busch wrote:
> > On Thu, Sep 08, 2022 at 09:45:26AM -0700, Keith Busch wrote:
> >> From: Keith Busch <kbusch@kernel.org>
> >>
> >> An iov length needs to be aligned to the logical block size, which may
> >> be larger than the memory alignment.
> > 
> > [cc'ing some other interested folks]
> > 
> > Any thoughts on this patch? It is fixing an observed IO error  when running
> > virtio-blk with the default 512b logical block size backed by a drive 
> > formatted
> > with 4k logical block.
> 
> The patch look OK to me, but having virtio expose a 512B LBA size for a 
> backing
> device that has 4K LBAs will break all IOs if caching is turned off (direct 
> IOs
> case), even if this patch is applied. No ?

Oh, as to why that type of setup "works" with O_DIRECT, when the check below
returns 'false', qemu allocates a bounce buffer. We want that to happen if the
guest's virtio driver tries to read/write 512b. The lengths just need to be
checked against the backing store's block size instead of the memory address
alignment.

> >> @@ -3243,13 +3243,14 @@ bool bdrv_qiov_is_aligned(BlockDriverState *bs, 
> >> QEMUIOVector *qiov)
> >>  {
> >>      int i;
> >>      size_t alignment = bdrv_min_mem_align(bs);
> >> +    size_t len = bs->bl.request_alignment;
> >>      IO_CODE();
> >>  
> >>      for (i = 0; i < qiov->niov; i++) {
> >>          if ((uintptr_t) qiov->iov[i].iov_base % alignment) {
> >>              return false;
> >>          }
> >> -        if (qiov->iov[i].iov_len % alignment) {
> >> +        if (qiov->iov[i].iov_len % len) {
> >>              return false;
> >>          }
> >>      }