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

[Alex Bennée]

Markus Armbruster <address@hidden> writes:

> Alex Bennée <address@hidden> writes:
>
>> 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 <address@hidden>
>>
>> ---
>> 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.
>> +         */
>> +        uint64_t backing_len = blk_getlength(pfl->blk);
>> +
>> +        if (backing_len < total_len) {
>> +            if (pfl->ro) {
>> +                memset(pfl->storage, 0, total_len);
>> +                total_len = backing_len;
>> +            } else {
>> +                error_setg(errp, "device(s) needs %" PRIu64 " bytes, "
>> +                           "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);
>
> Cases:
>
> * (MATCH) If the image size matches the device size: accept
>
>   Good.
>
> * (SHORT-RO): If the image is smaller than the device, and the device is
>   read-only: accept, silently pad to device size.
>
>   New convenience feature to save you the trouble of padding the image.
>   Personally, I'm wary of such conveniences; I'd rather force users to
>   be explicit about their intent.  Advice, not objection.

It's more a porting of existing behaviour with -bios to the "new" way of
properly specifying flash. That said I'd happily report a warning to the
user to say that's what we have done but I'm unsure what the best way to
do that is. I messed around with warn_report but it seems to be designed
for adding notifications to existing error failures so doesn't work
unless *errp points at something. Falling back to an fprintf doesn't
seem right.

>
> * (SHORT-RW): If the image is smaller than the device, and the device is
>   read/write: reject.
>
>   Good.  The alternative would be "padding, and writes to the padded
>   area aren't actually persistent", but that would be awful.
>
> * (LONG) If the image is larger than the device: accept, silently ignore
>   the image's extra bytes.
>
>   I know this is what we've always done, but that doesn't make it a good
>   idea.  What's the use case for silently truncating firmware images?
>   Other than giving users yet another way to create guests that
>   perplexingly fail to boot.

Again I'd happily be a bit noisier to the user here but not stop what
already worked from continuing to do so.

--
Alex Bennée