Re: hvm crash on hypercall event channel

[Ian Campbell <Ian.Campbell@citrix.com>]

On Wed, 2012-04-11 at 13:02 +0100, Daniel Castro wrote:
> On Tue, Apr 10, 2012 at 9:13 PM, Tim Deegan <tim@xen.org> wrote:
> > At 20:30 +0900 on 10 Apr (1334089815), Daniel Castro wrote:
> >> Hello All,
> >>
> >> I am writing the PV-Drivers for Seabios.
> >>
> >> When I put a request on the front ring and issue the hypercall to
> >> notify, the hvm guest crashes.
> >>
> >> Here is the dmesg output:
> >>
> >> (XEN) realmode.c:116:d10 Failed to emulate insn.
> >> (XEN) realmode.c:166:d10 Real-mode emulation failed @ f000:00001c4b:
> >> 0f aa ba b2 00 ec
> >
> > 0F AA is RSM, which is a pretty surprising instruction to find in a
> > hypercall invocation -- or indeed anywhere outside machine-specific SMM
> > code.  Is there SMM code in SeaBIOS?  It may be that you've ended up
> > jumping to a misaligned instruction boundary.
> >
> >> Nothing out of the ordinary. Except that the hypercall is issued under
> >> 16bit, It works under 32bit.
> >
> > Are you using the hypercall page to make your hypercall?  Its contents
> > don't make sense in 16-bit mode, only in 32-bit and 64-bit.  Since the
> > register arguments are 32-bit anyway you might want to make all your
> > hypercalls from 32-bit code anyway; otherwise you'll need to make your
> > own 16-bit stubs, with the right prefixes for the MOV imm32.
> 
> I have no idea how to fix this :(

It's looking likely that you'll have to go to 32 bit mode to do all of
the actual I/O associated with the PV devices. That ought to simplify a
bunch of stuff WRT handling the rings etc too.

Ian.