Re: [Qemu-devel] [PATCH v3] hw/block: better reporting on pflash backing file mismatch

[Laszlo Ersek <lersek@redhat.com>]

On 02/21/19 19:48, Alex Bennée wrote:
> It looks like there was going to be code to check we had some sort of
> alignment so lets replace it with an actual check. This is a bit more
> useful than the enigmatic "failed to read the initial flash content"
> when we attempt to read the number of bytes the device should have.
> 
> This is a potential confusing stumbling block when you move from using
> -bios to using -drive if=pflash,file=blob,format=raw,readonly for
> loading your firmware code. To mitigate that we automatically pad in
> the read-only case.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> 
> ---
> v3
>   - tweak commit title/commentary
>   - use total_len instead of device_len for checks
>   - if the device is read-only do the padding for them
>   - accept baking_len > total_len (how to warn_report with NULL *errp?)
> ---
>  hw/block/pflash_cfi01.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/block/pflash_cfi01.c b/hw/block/pflash_cfi01.c
> index 00c2efd0d7..37d7513c45 100644
> --- a/hw/block/pflash_cfi01.c
> +++ b/hw/block/pflash_cfi01.c
> @@ -714,13 +714,6 @@ static void pflash_cfi01_realize(DeviceState *dev, Error **errp)
>      }
>      device_len = sector_len_per_device * blocks_per_device;
>  
> -    /* XXX: to be fixed */
> -#if 0
> -    if (total_len != (8 * 1024 * 1024) && total_len != (16 * 1024 * 1024) &&
> -        total_len != (32 * 1024 * 1024) && total_len != (64 * 1024 * 1024))
> -        return NULL;
> -#endif
> -
>      memory_region_init_rom_device(
>          &pfl->mem, OBJECT(dev),
>          &pflash_cfi01_ops,
> @@ -747,6 +740,27 @@ static void pflash_cfi01_realize(DeviceState *dev, Error **errp)
>      }
>  
>      if (pfl->blk) {
> +        /*
> +         * Validate the backing store is the right size for pflash
> +         * devices. It should be padded to a multiple of the flash
> +         * block size. If the device is read-only we can elide the
> +         * check and just null pad the region first. If the user
> +         * supplies a larger file we silently accept it.

(1) I recommend adding "and ignore the tail".

> +         */
> +        uint64_t backing_len = blk_getlength(pfl->blk);

(2) Didn't we intend to check for blk_getlength() errors (or assert that
there would be none)?

> +
> +        if (backing_len < total_len) {
> +            if (pfl->ro) {
> +                memset(pfl->storage, 0, total_len);

(3) Should we "optimize" (well, okay, de-pessimize) this to:

  memset((uint8_t*)pfl->storage + backing_len, 0,
         total_len - backing_len);

?

> +                total_len = backing_len;
> +            } else {
> +                error_setg(errp, "device(s) needs %" PRIu64 " bytes, "

(4) not too important, I'm just curious: why the optional plural?

> +                           "backing file provides only %" PRIu64 " bytes",
> +                           total_len, backing_len);
> +                return;
> +            }
> +        }
> +
>          /* read the initial flash content */
>          ret = blk_pread(pfl->blk, 0, pfl->storage, total_len);
>  
> 

I don't feel too strongly about these, so if you disagree, I won't push.

Thanks!
Laszlo