Re: hvm crash on hypercall event channel

[Tim Deegan <tim@xen.org>]

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.

Cheers,

Tim.