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

[Kevin Wolf]

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.

> +            ret = blk_pread(blk, offset, bytes,
>                               (uint8_t *) buf + offset, 0);

blk_*() makes even more sense conceptually, but it should behave the
same.

Kevin