On 2017/11/20 18:13, Greg Kurz wrote: > On Mon, 20 Nov 2017 13:48:59 +0800 > jiangyiwen <jiangyi...@huawei.com> wrote: > >> Now v9fs in linux has already supported O_DIRECT(v9fs_direct_IO), >> when guest user open file with O_DIRECT flag and return success, >> so user hopes data doesn't pass through page cache, but 9pfs in >> qemu ignore direct disk access and use host page cache, it is not >> match to DIRECT_IO semantic, so we should not ignore O_DIRECT in >> 9pfs unless v9fs in linux don't support DIRECT_IO. >> >> And if server fs don't support O_DIRECT, user will receive -EINVAL >> and know the filesystem don't support O_DIRECT. >> >> So in order to ensure semantic consistency, don't ignore direct >> disk access in 9pfs. >> > > There are good reasons for us to ignore O_DIRECT. AFAIK, O_DIRECT requires > the application to take care of the alignment of the buffers with either the > logical block size of the filesystem (linux <= 2.4) or the logical block size > of the underlying storage... I don't really see how you can achieve that since > the linux 9p client only cares to break up requests into msize-chunks, and > ignores alignment. ie, you're likely to not ensure semantic consistency on the > host side anyway and linux will fallback to cached I/O. > > Also, this change would silently break existing users of O_DIRECT, which isn't > acceptable... it would require some fsdev property to enable this new > behavior. >
Thank you very much, I understand what your mean, v9fs in linux doesn't take care of the alignment, only split as the msize. Then there is another problem, should v9fs support DIRECT_IO? or delete DIRECT_IO ops in v9fs? Thanks, Yiwen. >> Signed-off-by: Yiwen Jiang <jiangyi...@huawei.com> >> --- >> hw/9pfs/9p.c | 4 ---- >> 1 file changed, 4 deletions(-) >> >> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c >> index 52d4663..5ea01c4 100644 >> --- a/hw/9pfs/9p.c >> +++ b/hw/9pfs/9p.c >> @@ -155,10 +155,6 @@ static int get_dotl_openflags(V9fsState *s, int oflags) >> */ >> flags = dotl_to_open_flags(oflags); >> flags &= ~(O_NOCTTY | O_ASYNC | O_CREAT); >> - /* >> - * Ignore direct disk access hint until the server supports it. >> - */ >> - flags &= ~O_DIRECT; >> return flags; >> } >> > > > . >
