From: Maxim Levitsky <firstname.lastname@example.org>
To: Paolo Bonzini <email@example.com>,
Christoph Hellwig <firstname.lastname@example.org>, Keith Busch <email@example.com>
Cc: firstname.lastname@example.org, email@example.com,
Kevin Wolf <firstname.lastname@example.org>,
Michael Roth <email@example.com>
Subject: Re: Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures
Date: Sun, 02 Oct 2022 11:59:42 +0300 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On Thu, 2022-09-29 at 19:35 +0200, Paolo Bonzini wrote:
> On 9/29/22 18:39, Christoph Hellwig wrote:
> > On Thu, Sep 29, 2022 at 10:37:22AM -0600, Keith Busch wrote:
> > > > I am aware, and I've submitted the fix to qemu here:
> > > >
> > > > https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00398.html
> > >
> > > I don't think so. Memory alignment and length granularity are two completely
> > > different concepts. If anything, the kernel's ABI had been that the length
> > > requirement was also required for the memory alignment, not the other way
> > > around. That usage will continue working with this kernel patch.
Yes, this is how I also understand it - for example for O_DIRECT on a file which
resides on 4K block device, you have to use page aligned buffers.
But here after the patch, 512 aligned buffer starts working as well - If I
understand you correctly the ABI didn't guarantee that such usage would fail,
but rather that it might fail.
> > Well, Linus does treat anything that breaks significant userspace
> > as a regression. Qemu certainly is significant, but that might depend
> > on bit how common configurations hitting this issue are.
> Seeing the QEMU patch, I agree that it's a QEMU bug though. I'm
> surprised it has ever worked.
> It requires 4K sectors in the host but not in the guest, and can be
> worked around (if not migrating) by disabling O_DIRECT. I think it's
> not that awful, but we probably should do some extra releases of QEMU
> stable branches.
I must admit I am out of the loop on the exact requirements of the O_DIRECT.
If I understand that correctly, after the patch in question,
qemu is able to use just 512 bytes aligned buffer to read a single 4K block from the disk,
which supposed to fail but wasn't guarnteed to fail.
Later qemu it submits iovec which also reads a 4K block but in two parts,
and if I understand that correctly, each part (iov) is considered
to be a separate IO operation, and thus each has to be in my case 4K in size,
and its memory buffer *should* also be 4K aligned.
(but it can work with smaller alignement as well).
Assuming that I understand all of this correctly, I agree with Paolo that this is qemu
bug, but I do fear that it can cause quite some problems for users,
especially for users that use outdated qemu version.
It might be too much to ask, but maybe add a Kconfig option to keep legacy behavier
for those that need it?
next prev parent reply other threads:[~2022-10-02 9:00 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-29 15:41 Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures Maxim Levitsky
2022-09-29 15:48 ` Keith Busch
2022-09-29 16:16 ` Maxim Levitsky
2022-09-29 16:37 ` Keith Busch
2022-09-29 16:39 ` Christoph Hellwig
2022-09-29 17:35 ` Paolo Bonzini
2022-10-02 8:59 ` Maxim Levitsky [this message]
2022-10-02 13:56 ` Keith Busch
2022-10-03 7:06 ` Maxim Levitsky
2022-09-30 11:52 ` Thorsten Leemhuis
2022-11-04 11:59 ` Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures #forregzbot Thorsten Leemhuis
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).