Re: [Qemu-devel] [RFC PATCH] s390x-linux-user

[Paul Brook]

On Friday 26 June 2009, Blue Swirl wrote:
> On 6/26/09, Paul Brook <address@hidden> wrote:
> > On Friday 26 June 2009, Blue Swirl wrote:
> >  > On 6/26/09, Ulrich Hecht <address@hidden> wrote:
> >  > >  There is a very peculiar S/390 instruction called "EXECUTE". What
> >  > > it does is to take another instruction stored somewhere in memory,
> >  > > logical-OR the second byte of the instruction with the LSB of R0 and
> >  > > then execute the result, without changing the instruction in memory
> >  > > or the program counter. Any idea how to implement this in QEMU?
> >  > > Currently, I'm interpreting the couple of instructions that GCC uses
> >  > > EXECUTE with, but in the long run that would amount to implementing
> >  > > a second emulator...
> >  >
> >  > Maybe something like this: Make a special TB of the EXECUTE
> >  > instruction and add LSB of R0 to TB flags for these TBs. Then you can
> >  > examine R0, OR and generate code at translation time. The TBs linking
> >  > to EXECUTE TB may need to be special too in order to track for R0.
> >
> > That's not sufficient. The results also depend on the referenced
> > instruction.
>
> Then add the second byte of the referenced instruction to TB flags? Or
> maybe just the result of the OR operation for compactness?

No. You need the whole instruction. Which is fetched from memory, so is not 
easily available when you're checking TB flags.
To do it this way, I think you'd need to split the instruction in two. The 
first part would load the whole instruciton from memory, or with r0, then 
store the result in an internal CPU pseudo-register to the whole instruction, 
and cuse annother TB lookup. The second would generate code that cleared the 
pseudo-register then executed the code that was stored in it.
You'd have to include the whole of the pseudo-register in TB_FLAGS, and I 
doubt you've got enough bits for that.

OTOH, tweaking the TCG interface so that it works as an interpreter shouldn't 
be all that hard. It's something I've been considering to do for a while, and 
would mean that you can build both interpreter and translator from the same 
source.

Paul