Re: [PULL 03/38] pflash: Only read non-zero parts of backend image

[Cédric Le Goater]

On 2/7/23 13:48, Kevin Wolf wrote:
> Am 07.02.2023 um 10:19 hat Cédric Le Goater geschrieben:
> > On 2/7/23 09:38, Kevin Wolf wrote:
> > > Am 06.02.2023 um 16:54 hat Cédric Le Goater geschrieben:
> > > > On 1/20/23 13:25, Kevin Wolf wrote:
> > > > > From: Xiang Zheng <zhengxiang9@huawei.com>
> > > > >
> > > > > Currently we fill the VIRT_FLASH memory space with two 64MB NOR images
> > > > > when using persistent UEFI variables on virt board. Actually we only use
> > > > > a very small(non-zero) part of the memory while the rest significant
> > > > > large(zero) part of memory is wasted.
> > > > >
> > > > > So this patch checks the block status and only writes the non-zero part
> > > > > into memory. This requires pflash devices to use sparse files for
> > > > > backends.
> > > > >
> > > > > Signed-off-by: Xiang Zheng <zhengxiang9@huawei.com>
> > > > >
> > > > > [ kraxel: rebased to latest master ]
> > > > >
> > > > > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > > > > Message-Id: <20221220084246.1984871-1-kraxel@redhat.com>
> > > > > Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> > > > > Reviewed-by: Kevin Wolf <kwolf@redhat.com>
> > > > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > >
> > > > This newly merged patch introduces a "regression" when booting an Aspeed
> > > > machine. The following extra m25p80 patch (not yet merged) is required
> > > > for the issue to show:
> > > >
> > > >     https://lore.kernel.org/qemu-devel/20221115151000.2080833-1-clg@kaod.org/
> > > >
> > > > U-Boot fails to find the filesystem in that case.
> > > >
> > > > It can be easily reproduced with the witherspoon-bmc machine and seems
> > > > to be related to the use of a UBI filesystem. Other Aspeed machines not
> > > > using UBI are not impacted.
> > > >
> > > > Here is a tentative fix. I don't know enough the block layer to explain
> > > > what is happening :/
> > >
> > > I was puzzled for a moment, but...
> > >
> > > > @@ -39,7 +39,7 @@ static int blk_pread_nonzeroes(BlockBack
> > > >                return ret;
> > > >            }
> > > >            if (!(ret & BDRV_BLOCK_ZERO)) {
> > > > -            ret = bdrv_pread(bs->file, offset, bytes,
> > >
> > > 'bs->file' rather than 'bs' really looks wrong. I think replacing that
> > > would already fix the bug you're seeing.
> > >
> > > Just to be sure, how did you configure the block backend? bs->file would
> > > happen to work more or less with raw over file-posix (which is probably
> > > what Gerd tested), but I think it breaks with anything else.
> >
> > The command is  :
> >
> >    $ qemu-system-arm -M witherspoon-bmc -net user \
> >         -drive file=/path/to/file.mtd,format=raw,if=mtd \
> >         -nographic -serial mon:stdio -snapshot
> >
> > If I remove '-snapshot', all works fine.
>
> Ok, that makes sense then. -snapshot creates a temporary qcow2 overlay,
> and then what your guest sees with bs->file is not the virtual disk
> content of the qcow2 image, but the qcow2 file itself.

yes. Same symptom with pflash devices, TCG and KVM. The guest hangs with 
-snapshot.

C.

qemu-system-aarch64 -M virt -smp 2 -cpu max -accel tcg,thread=multi -nographic 
-m 2G -drive 
if=pflash,format=raw,file=/usr/share/edk2/aarch64/QEMU_EFI-silent-pflash.raw,readonly=on
 -drive if=pflash,format=raw,file=rhel9-varstore.img -device 
virtio-net,netdev=net0,bus=pcie.0,addr=0x3 -netdev user,id=net0 -drive 
file=rhel9-arm64.qcow2,if=none,id=disk,format=qcow2,cache=none -device 
virtio-blk-device,drive=disk -serial mon:stdio -snapshot