re: [Qemu-devel] Host API escape

re: [Qemu-devel] Host API escape

[EricNorthup]

On Mon, 13 Sep 2004 23:45:07 +0100, Derek Fawcus wrote:

>On Mon, Sep 13, 2004 at 11:16:14PM +0200, Fabrice Bellard wrote:
>> Derek Fawcus wrote:
>> > I was adding a fake instruction
>
>So will you at all interested in this bit,  want to wait for it to
>cook some more first,  or not interested?
>
>It's basically a "new" instruction,  using opcodes that should normally
>generate undefined/illegal opcode traps,  hence it should still be
>usable
>(in some form) if/when doing fast/native x86-on-x86.  The point
> being to provide a a controlled escape to talk with the host -
therefore it's
> called "hostapi"...
[. . .]

Ok, I think the things you can achieve with this mechanism are very
exciting.  But please, *please* do not make another fork in x86!

What if you use the WRMSR instructions instead?  Then you can define
MSRs which have certain behavior when written to / read from.  The whole
point of MSRs is that they are model-specific, and that is the way to do
this compatibly.

Just a suggestion

--Eric

Re: [Qemu-devel] Host API escape

[Derek Fawcus]

On Mon, Sep 13, 2004 at 10:43:00PM -0400, EricNorthup wrote:
> Ok, I think the things you can achieve with this mechanism are very
> exciting.  But please, *please* do not make another fork in x86!

Huh?  What do you mean by "another fork in x86"?  Or are you simply
referring to the fact that I've (re)defined a special instruction?

> What if you use the WRMSR instructions instead?  Then you can define
> MSRs which have certain behavior when written to / read from.  The whole
> point of MSRs is that they are model-specific, and that is the way to do
> this compatibly.

Hmm - a possibility.

I can look at changing to that once I've got the redir stuff working.
My initial concern was actually not to use / change any target visible
register,  so that I can inject this anywhere as a debug hook - where
it's currently quite useful.

DF

[Qemu-devel] Re: Host API escape

[Ben Pfaff]

Derek Fawcus <dfawcus@cisco.com> writes:

> On Mon, Sep 13, 2004 at 10:43:00PM -0400, EricNorthup wrote:
>> What if you use the WRMSR instructions instead?  Then you can define
>> MSRs which have certain behavior when written to / read from.  The whole
>> point of MSRs is that they are model-specific, and that is the way to do
>> this compatibly.
>
> Hmm - a possibility.

For what it's worth, VMware products use a magic I/O port for
guest<->host communication according to
http://chitchat.at.infoseek.co.jp/vmware/backdoor.html
-- 
Ben Pfaff 
email: blp@cs.stanford.edu
web: http://benpfaff.org