Re: hvm crash on hypercall event channel

[Daniel Castro <evil.dani@gmail.com>]

On Wed, Apr 11, 2012 at 9:06 PM, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> 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.

What are my choices?
1. Add support for 16bit hypercalls on Xen.
2. Jump to 32 bit
3. make a 16bit "special" hypercall on SeaBIOS
4. Any other choice?

Thanks to all the comments.

>
> Ian.
>
>
>



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