Re: [Qemu-devel] [PATCH v6 3/6] tests: in IDE and AHCI tests perform DMA write before flushing

[Evgeny Yakovlev <eyakovlev@virtuozzo.com>]

On 15.07.2016 20:23, Eric Blake wrote:
> On 07/15/2016 02:08 AM, Evgeny Yakovlev wrote:
>
>>>> + * Write sector 0 with random data to make AHCI storage dirty
>>> If we ever have a case where we open a disk without specifying -raw, the
>>> random data _might_ resemble some other format and cause probe to
>>> misbehave; as such, we also have code in the block layer that
>>> specifically prevents writes to sector 0 for some data. Should you pick
>>> a different sector than 0, so as to avoid any (remote) possibility that
>>> the random data could change probe results or be rejected?
>>>
>> Not sure if i understand the problem you're referring to here. Those are
>> blkdebug tests, those disks are created, emulated with blkdebug backend,
>> flushed and then thrown away. So is there really any possibility for
>> reopening the image and accidentally parsing a partition table in sector 0?
>>
>> Also, not sure what you mean by "code in the block layer that
>> specifically prevents writes to sector 0 for some data". Can you explain
>> that bit, because it sounds pretty scary. How can we deny guest VM to
>> write anything to sector 0 on its emulated disk?
> Read block/raw_bsd.c:raw_co_writev_flags() for the gory details.  If the
> guest ever gets a raw format driver because the user forgot to say
> '--format $foo', then we prevent the guest from writing anything into
> sector 0 that would be probed as non-raw.  It means there are only a
> handful of patterns that the guest cannot write into the first sector,
> but it IS a non-zero number of patterns.  How the guest behaves if such
> a write is attempted depends on the error policy you have on that
> device; it might show up as an EIO error to the guest, or it might stop
> the guest from executing and raise a qemu event to the management
> application, but the point is that we actively prohibit some writes to
> sector 0 on a probed raw disk.  Using any sector other than 0 doesn't
> have this limitation, or you can ensure that your test ALWAYS passes the
> appropriate --format $foo so that the disk is never probed as another
> way to avoid limitations on sector 0.
>
>

I think i get it now. Thanks!