linux-kernel - Re: Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures

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Date:   Thu, 29 Sep 2022 19:35:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, qemu-devel@...gnu.org,
        kvm@...r.kernel.org, Kevin Wolf <kwolf@...hat.com>,
        Michael Roth <mdroth@...ux.vnet.ibm.com>
Subject: Re: Commit 'iomap: add support for dma aligned direct-io' causes
 qemu/KVM boot failures

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.
> 
> 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.

Paolo