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Re: Syncing CPU caches from userland on MIPS

To: Aurelien Jarno <aurelien@aurel32.net>
Subject: Re: Syncing CPU caches from userland on MIPS
From: David Daney <ddaney@caviumnetworks.com>
Date: Tue, 24 Nov 2009 13:04:55 -0800
Cc: linux-mips@linux-mips.org, Arnaud Patard <arnaud.patard@rtp-net.org>
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Aurelien Jarno wrote:
Hi all,

This question is not really kernel related, but still MIPS related, I
hope you don't mind.

Arnaud Patard and myself are trying to get qemu working on MIPS [1],
which includes translating TCG code (internal representation) into MIPS
instructions, that are then executed. Most of the code works, but we have some strange behaviors that seems related to CPU caches.

The code is written to a buffer, which is then executed. Before the
execution, the caches are synced using the cacheflush syscall:

| #include <sys/cachectl.h>
| | | static inline void flush_icache_range(unsigned long start, unsigned long stop)
| {
|     cacheflush ((void *)start, stop-start, ICACHE);
| }

It seems this is not enough, as sometimes, some executed code does not
correspond to the assembly dump of this memory region. This seems to be especially the case of memory regions that are written twice, due to
relocations:
1) a branch instruction is written with an offset of 0
2) the offset is patched

Try inserting an 'asm volatile ("sync" ::: "memory");' here. If that fixes things, then we can assume that your cacheflush system call is buggy, and would need to add a sync.

3) cacheflush is called

Sometimes the executed code correspond to the code written in 1), which
means the branch is skipped.

Does someone knows and/or has example code to correctly sync the CPU caches from userland on MIPS?


http://gcc.gnu.org/viewcvs/trunk/libffi/src/mips/ffi.c

Look at ffi_prep_closure_loc() around line 721.


We also do a similar thing in the kernel in flush_cache_sigtramp(), but that is not really userland.


David Daney

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