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

From: "Cédric Le Goater" <clg@kaod.org>
To: Kevin Wolf <kwolf@redhat.com>,
	"Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc: qemu-block@nongnu.org, peter.maydell@linaro.org,
	qemu-devel@nongnu.org, "Daniel P. Berrangé" <berrange@redhat.com>,
	kraxel@redhat.com
Subject: Re: [PULL 03/38] pflash: Only read non-zero parts of backend image
Date: Tue, 7 Mar 2023 15:15:30 +0100	[thread overview]
Message-ID: <afed603f-8807-8e50-53cc-604a7c7cf8e5@kaod.org> (raw)
In-Reply-To: <ZAdDbe+B/ulQdkFI@redhat.com>

On 3/7/23 15:00, Kevin Wolf wrote:
> Am 03.03.2023 um 23:51 hat Maciej S. Szmigiero geschrieben:
>> On 8.02.2023 12:19, Cédric Le Goater wrote:
>>> 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
>>>
>>>
>>>
>>
>> +1 here for QEMU + KVM/x86: OVMF CODE file fails to load (is all zeroes)
>> with either "-snapshot" QEMU command line option or even with just "snapshot=on"
>> setting enabled on pflash0.
>>
>> Reverting this patch seems to fix the issue.
> 
> Hm, so we know the fix, but nobody has submitted it as an actual patch?

Sorry. I thought the solution was more complex and got pulled in other
tasks ...

> I'll send one...

Thanks,

C.