Re: [Qemu-devel] RFC: Code fetch optimisation

[Paul Brook <paul@codesourcery.com>]

On Sunday 14 October 2007, J. Mayer wrote:
> Here's an updated version of the code fetch optimisation patch against
> current CVS.
> As a remainder, this patch avoid use of softmmu helpers to fetch the
> code in most case. A new target define TARGET_HAS_VLE_INSNS has been
> added which is used to handle the case of an instruction that span 2
> pages, when the target CPU uses a variable-length instructions encoding.
> For pure RISC, the code fetch is done using raw access routines.

> +    unsigned long phys_pc;
> +    unsigned long phys_pc_start;

These are ram offsets, not physical addresses. I recommend naming them as such 
to avoid confusion.

> +    opc = glue(glue(lds,SUFFIX),MEMSUFFIX)(virt_pc);
> +    /* Avoid softmmu access on next load */
> +    /* XXX: dont: phys PC is not correct anymore
> +     *      We could call get_phys_addr_code(env, pc); and remove the else
> +     *      condition, here. 
> +     */
> +    //*start_pc = phys_pc;

The commented out code is completely bogus, please remove it. The comment is 
also somewhat misleading/incorrect. The else would still be required for 
accesses that span a page boundary.

The code itself looks ok, though I'd be surprised if it made a significant 
difference. We're always going to hit the fast-path TLB lookup case anyway.

Paul