On 2022/09/13 15:51, Keith Busch wrote:
> 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.
Ah ! Got it. Thanks !
>
>>>> @@ -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;
>>>> }
>>>> }
--
Damien Le Moal
Western Digital Research
