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Re: HVM disc corruption (was Re: Xen won't start its device backend?)

Hi folks,

On Tue, Jun 02, 2015 at 11:46:43PM +0200, Manuel Bouyer wrote:
> > > Yes, it looks like the IDE emulation in qemu is not working as it should
> > > with LBA48
> > As per suggestion on chat, i changed back to xentools41 and had to revert back
> > to xenkernel41 too. It feels more responsive but it still has the same mapping
> > issue, so far no luck. What strikes me is that it is near bit 28, a kind of
> > odd number unless the top 4 bits mean something else to code somewhere in the
> > whole chain.
> Yes, that's the IDE interface. The initial interface ony allowed 28 bits
> sectors addressing (the the 8bits sector register, the 16bits cylinder
> register and the 4bits head regiter - the other 4 bits are used for something
> else). Later the spec reused these registers for 28bit sector adressing.
> When drives larger than 128GB did show up, the spec was updated again by
> duplicating the LBA registers - writing the complete values requires 2
> registers writes (the first set of writes writes the high 24 bits,
> the second the low 24 bits).
> I suspect that qemu-dm doesn't properly emulate this two-step write,
> and the second set of writes just erases the previous write, effectively
> truncating the sector address to the low 24 bits
> (it works up to 128GB - 2^28 sectors - because the driver will use LBA
> and not LBA48 when possible). Fixing this will require a patch to qemu-dm.
> Maybe this has already been fixed in newer qemu versions ...

It sure isn't fixed in xenkernel45/xentools45 as it shows the same issue....
Why is it using the `traditional' qemu btw? This has the ide.c that isn't in
the newer qemu since those have an ide directory!


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