Re: [Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi <stefboombastic@gmail.com>]

On 01/24/2011 03:32 PM, Peter Maydell wrote:
>
> Being a JIT doesn't prohibit counting target instructions executed.
> It just means that counting them generally requires generating
> code to do the counting at runtime, so it's a more complicated
> change to make than it would be in a non-JIT emulator.
>
What do you mean? Should I change the code of qemu-user for counting the 
instructions, or should I add code into the target binaries?
> The major reason for not counting cycles is that for an emulation
> of a modern CPU this is pretty nearly impossible: the number
> of cycles an instruction takes can depend on whether it causes
> a cache miss, which CPU internal pipeline it uses, whether it
> needs to stall waiting for a result from an earlier insn, whether
> the CPU correctly predicted the branch leading up to it or not,
> and on and on. You would need to precisely model all the
> internals of each variant of each CPU, which would be a
> mammoth undertaking requiring probably unpublished internal
> data, and if you ever managed to finish it then it would run
> incredibly slowly and would probably contain enough bugs you
> couldn't trust the data it gave you anyway.
>
Yup, I think it was just a silly mistake of mine when in the first post 
I wrote cycles.. that was because for me anything that can estimate how 
long it takes to do the work would be fine.. I can't simply check the 
time because that is host machine dependent... Number of executed 
instructions would be fine..
>> This means that QEMU can
>> no longer run on a type of host it can't execute target code for
> This isn't correct; for instance there's hppa support in TCG for hppa
> hosts but no hppa target support, and there's sh4 target support
> but no TCG backend for it. The two ends are cleanly separated in
> qemu and don't generally depend on each other.
>
Well I experienced a strange behavior some time ago that initially made 
me think mr Rob was right on that though I knew host support and target 
support were separated in qemu: I tried to make directly qemu-ppc on a 
x86_64 machine from inside ppc-linux-user folder (i can do fine onto x86 
machine) and it failed because there was no tgc/x86_64/tcg_target.h, 
whereas doing the make from within the main folder worked.
So I do not understand very well.. is there some required headers fix 
when using the main make file?
  Best regards!
Stefano B.