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Re: [Qemu-block] [Qemu-devel] [PATCH v6 3/6] tests: in IDE and AHCI test

From: Evgeny Yakovlev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v6 3/6] tests: in IDE and AHCI tests perform DMA write before flushing
Date: Fri, 15 Jul 2016 20:44:42 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

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!

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