Re: [PATCH RESEND] KVM: when entering real mode align segment base to 16 bytes

Gleb Natapov
2010-12-27T14:58:44Z

On Mon, Dec 27, 2010 at 04:55:08PM +0200, Avi Kivity wrote:
> On 12/27/2010 04:37 PM, Gleb Natapov wrote:
> >> What's the scenario here? And what segment is involved? if it's fs
> >> or gs, or maybe ss, I can see it working out, but hardly otherwise.
> >>
> >The scenario is older Seabios that lefts DS segment unaligned during
> >switch to real mode in int1587 function (copy extended memory).
> >
>
> Did it not use %ds then?
>
It restores %ds before first use. There can be other guests of course
that try to use %ds in protected mode. We can distinguish one from the
other.

> What about switching back to protected mode? I see we do restore the
> base, so it can continue to use the segment. I guess this isn't an
> issue for 1587.
>
Correct. 1587 does not return to protected mode again.

--
Gleb.