From: Maxim Uvarov [mailto:maxim.uvarov@linaro.org] Sent: 04 October 2013 13:44 To: Ola Liljedahl Cc: linaro-networking@linaro.org; patches@linaro.org; linaro- kernel@lists.linaro.org Subject: Re: [LNG] [PATCH] LTP cacheflush01 add arm variant
On 10/04/2013 03:13 PM, Ola Liljedahl wrote:
What's the overhead of a system call compared to the cost of the actual
flush operation? Why are you asking about that? There should be some overhead but I don't think that somebody measured it.
[Ola] Because system calls are expensive and if we are only going to flush/invalidate a small amount of data but rather frequently (e.g. packet buffers in the days of incoherent I/O devices), doing this operation through a system call would degrade performance. This did look like a Linux system call.
What's the expected use case (specifically number of bytes flushed) of
this call? (Maybe this is completely out of our control). Well, it calls:
flush_cache_user_range(start, end);
coherent_user_range
- v7_coherent_user_range(start,end)
- Ensure that the I and D caches are coherent within specified
- region. This is typically used when code has been written to
- a memory region, and will be executed.
[Ola] Flush cache is a poor name for this call that merely assures synchronisation between I and D-caches (because instructions have been written by the application and should later be executed).
- start - virtual start address of region
- end - virtual end address of region
-EFAULT - virt address not mapped, -EINVAL - wrong start/end arguments, 0 - on success.
Original (mips only I suppose) test does not do any invalid parameters check. So I just added only right case.
This is a flush (and invalidate) operation, not a true invalidate
operation I assume, so it could be performed completely in user space, no supervisor privileges required. Is a user space implementation (using a DSO) contemplated? Did not understand that. What is "user space implementation using DSO" ?
[Ola] I meant vDSO. A kernel call actually implemented in user space using a shared object. This to avoid the overhead of trapping in and out of the kernel which could be substantial compared to the cost of flushing and invalidating a few cache lines in the I- and D-caches. That's why I ask about typical usage. Would this system call be called from e.g. JIT compilers?
A true invalidate operation (no writeback of dirty cache lines, any modified data would get lost) is more dangerous and are probably not allowed to be performed from user space. But the need for such an operation is negligible.
-- Ola
Thank you, Maxim.
-- Ola
Ola Liljedahl, Networking System Architect, ARM Telephone: +46 706 866 373 Skype: ola.liljedahl
-----Original Message----- From: Maxim Uvarov [mailto:maxim.uvarov@linaro.org] Sent: 03 October 2013 14:08 To: linaro-networking@linaro.org Cc: patches@linaro.org; linaro-kernel@lists.linaro.org; Maxim Uvarov Subject: [LNG] [PATCH] LTP cacheflush01 add arm variant
Cache flush on arm provided via __ARM_NR_cacheflush(unsigned long start, unsigned long end, int flags) defined in ./arch/arm/kernel/traps.c. Flags are not used, providing 0 there.
Signed-off-by: Maxim Uvarov maxim.uvarov@linaro.com
.../kernel/syscalls/cacheflush/cacheflush01.c | 9 +++++++++ 1 file changed, 9 insertions(+)
diff --git a/testcases/kernel/syscalls/cacheflush/cacheflush01.c b/testcases/kernel/syscalls/cacheflush/cacheflush01.c index 9c02f0a..fae51b4 100644 --- a/testcases/kernel/syscalls/cacheflush/cacheflush01.c +++ b/testcases/kernel/syscalls/cacheflush/cacheflush01.c @@ -137,6 +137,14 @@ int main(int ac, char **av) }
/* Invokes cacheflush() with proper parameters */+#if defined(__arm__)
TEST(ltp_syscall(__ARM_NR_cacheflush, addr, addr + getpagesize(),0));
if (TEST_RETURN == 0) {tst_resm(TPASS, "passed with no errno");} else {tst_resm(TFAIL, "failed with unexpected errno");}+#else TEST(ltp_syscall(__NR_cacheflush, addr, getpagesize(), ICACHE)); if (TEST_RETURN == 0) { tst_resm(TPASS, "passed with no errno"); @@ -157,6 +165,7 @@ int main(int ac, char **av) } else { tst_resm(TFAIL, "failed with unexpected errno"); } +#endif
cleanup(); tst_exit();-- 1.7.9.5
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