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

[Alex Bennée]

Laszlo Ersek <address@hidden> writes:

> 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 <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.
>
> (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)?

Oops, yes I'll fix that.

>
>> +
>> +        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);
>
> ?

I mean in the grand scheme of things it's unlikely to show up in any
benchmarks so I went for simple and easy to get right.

>
>> +                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?

I discovered the difference between device_len and total_len and found
(for some reason) the efivars came out as multiple devices.

>
>> +                           "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


--
Alex Bennée