[Qemu-devel] dyngen_code in 16 bit

[Qemu-devel] dyngen_code in 16 bit

[Clemens Kolbitsch]

Hi!
For a research project I extended Qemu to include some extra code inside the 
op_XXX instructions that increased the generated TB-code's size to quite some 
extend... 

Now I have a problem when having block chaining enabled (that I don't want to 
disable for performance reasons :-/): The code_gen_buffer sometimes contains 
code areas that span more than 0xffff bytes, however, dyngen and all 
functions related to it use 16 bit pointers, etc. Therefore, e.g. the 
dyngen_code function uses the 16 bit pointers to overwrite certain params and 
of course destroys the TB-code.

When working with x86 hosts and guests (both 32 bit), is there a specific 
reason for all these pointers to be 16 bits or has it just been a safe 
assumption up to now?? I have tried rewriting the code to use 32 bit, but 
keep getting segfaults... however, of course, I might have missed some code 
still.

Any help is - as always - greatly appreciated!!

Cheers,
Clemens

Re: [Qemu-devel] dyngen_code in 16 bit

[Paul Brook]

On Monday 14 April 2008, Clemens Kolbitsch wrote:
> Hi!
> For a research project I extended Qemu to include some extra code inside
> the op_XXX instructions that increased the generated TB-code's size to
> quite some extend...

Don't do that. Put it in a helper function. The performance overhead will be 
minimal.

Most likely the segfaults are because you're overrunning a buffer. qemu has 
static limits on the maximum size of an guest instruction.

Paul

[Qemu-devel] dyngen_code in 16 bit

[Clemens Kolbitsch]

Hi!
For a research project I extended Qemu to include some extra code inside the 
op_XXX instructions that increased the generated TB-code's size to quite some 
extend... 

Now I have a problem when having block chaining enabled (that I don't want to 
disable for performance reasons :-/): The code_gen_buffer sometimes contains 
code areas that span more than 0xffff bytes, however, dyngen and all 
functions related to it use 16 bit pointers, etc. Therefore, e.g. the 
dyngen_code function uses the 16 bit pointers to overwrite certain params and 
of course destroys the TB-code.

When working with x86 hosts and guests (both 32 bit), is there a specific 
reason for all these pointers to be 16 bits or has it just been a safe 
assumption up to now?? I have tried rewriting the code to use 32 bit, but 
keep getting segfaults... however, of course, I might have missed some code 
still.

Any help is - as always - greatly appreciated!!

Cheers,
Clemens