Re: [Xen-devel] Loading a 32 bit kernel from 64 bit grub-xen

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-07-08 at 11:37 -0400, Phillip Susi wrote:
> On 7/2/2014 5:26 AM, Ian Campbell wrote:
> 
> So you are saying

I don't know, because you've helpfully trimmed all of the context of
something I said a week ago.

/me consults list archive, I think you are responding to:

> > Running a 32-bit userspace under a 64-bit kernel is a very different
> > thing to switching a kernel from 64-bit down to 32-bit (or vice versa).
> >
> > While the former is of course supported just fine the latter is not and
> > IIRC doing so would be very difficult for a PV kernel due to the way the
> > interfaces are constructed (e.g. to p2m entries change size, the maximum
> > addressable machine address changes).

----

>  that you could put the cpu into 32 bit mode, but xen
> would still believe that the domU is 64 bit, and thus, expect the
> hypercalls to be 64 bit?

Assuming you are asking about 32-bit kernel mode then a Xen PV 64-bit
guest cannot be transitioned to run in kernel mode in anything other
than 64-bit mode. There is simply no way to put the cpu into 32 bit mode
while remaining in kernel mode.

If you meant in userspace then all userspace hypercalls are made via a
kernel driver and thus appear to come from the 64-bit kernel and Xen
sees them as such.

Ian.


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