Re: [bisected] QEMU guest boot failure since 6.0 on x86_64 host

[Dmitrii Tcvetkov <me@demsh.org>]

On Wed, 19 Oct 2022 19:28:17 -0600
Keith Busch <kbusch@kernel.org> wrote:

> On Thu, Oct 20, 2022 at 03:17:25AM +0300, Dmitrii Tcvetkov wrote:
> > 
> > Bisect led me to commit b1a000d3b8ec5  ("block: relax direct io
> > memory alignment"). I was unable to resolve revert conflicts when
> > tried to revert b1a000d3b8ec5  ("block: relax direct io memory
> > alignment") as I lack necessary understanding of block subsystem.
> 
> Background info: when your virtual block device's logical block size
> is smaller than the host's block device backing it, qemu needs to
> bounce unaligned buffers when using direct-io.
> 
> Historically for direct-io, the logical block size happened to also be
> the memory page offset alignment. QEMU did this the other way around:
> it used the memory offset as the block size, and that was not
> intended:
> 
>   https://lore.kernel.org/lkml/32db4f89-a83f-aac4-5d27-0801bdca60bf@redhat.com/
> 
> The kernel patch you bisected to detangled memory alignment from
> logical block size, so now older qemu versions have the wrong idea of
> the minimum vector size. That is fixed in the qemu repository here:
> 
>   https://git.qemu.org/?p=qemu.git;a=commitdiff;h=25474d90aa50bd32e0de395a33d8de42dd6f2aef
> > 
> > This fails to boot on 6.0+ host:
> > # losetup -b 4096 -f image.raw
> > # qemu-system-x86_64 -enable-kvm -drive
> > file=/dev/loop0,format=raw,cache=none
> 
> In the above, your backing storage is 4k, and the default virtual
> device block size is 512b, so qemu needs to bounce that, but older
> versions might not do that as intended.
> 
> It should work if you include logical_block_size=4096 to the -drive
> parameters.
> 
> > These boot fine on 6.0+ host:
> > # losetup -b 4096 -f image.raw
> > # qemu-system-x86_64 -enable-kvm -drive
> > file=/dev/loop0,format=raw
> 
> The above is using cache, which doesn't have any alignment and size
> constraints, so works with anything sizes.
>  
> > # losetup -f image.raw
> > # qemu-system-x86_64 -enable-kvm -drive
> > file=/dev/loop0,format=raw,cache=none
> 
> The above is using a 512b formated backing store to a 512b emulated
> drive, so the matching means qemu never needs to bounce.

Thanks! Specifying logical_block_size=4096 indeed helps, guest still
doesn't boot but because it has partition table with an assumption of 512
sectors. After reinstall with logical_block_size=4096 specified it
boots.