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 <kbu...@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;
> >> }
> >> }
