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


From: Fabrice Bellard
Subject: Re: [Qemu-devel] RFC: Code fetch optimisation
Date: Fri, 12 Oct 2007 20:36:32 +0200
User-agent: Thunderbird 1.5.0.9 (X11/20070212)

Blue Swirl wrote:
On 10/12/07, J. Mayer <address@hidden> wrote:
Here's a small patch that allow an optimisation for code fetch, at least
for RISC CPU targets, as suggested by Fabrice Bellard.
The main idea is that a translated block is never to span over a page
boundary. As the tb_find_slow routine already gets the physical address
of the page of code to be translated, the code translator could then
fetch the code using raw host memory accesses instead of doing it
through the softmmu routines.
This patch could also be adapted to RISC CPU targets, with care for the
last instruction of a page. For now, I did implement it for alpha, arm,
mips, PowerPC and SH4.
I don't actually know if the optimsation would bring a sensible speed
gain or if it will be absolutelly marginal.

Please comment.

This will not work correctly for execution of MMIO registers, but
maybe that won't work on real hardware either. Who cares.

It can never happen because QEMU currently does not support it (see get_phys_addr_code()). I started to implement it but never really finished it (real hardware can do it so QEMU should support it). The idea consist in using a reserved ram page to store the code. Another point is that the TB must be discarded once executed as the MMIO data can change.

Regards,

Fabrice.




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