Hi!
There is a per core register on modern POWER processors (POWER6+) called
"DSCR": The DSCR (aka Data Stream Control Register) is supported on some
server PowerPC chips and allow some control over the prefetch of data
streams.
Its "per process" support has been added by
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=efcac6589a277c10060e4be44b9455cf43838dc1
"powerpc: Per process DSCR + some fixes (try#4)"
The patch adds the DSCR value into "struct thread_struct" and
saves/restores it when the context is switched. Also, there is some policy
to initialize the register when a thread starts. So far so good.
Here is the problem.
Some customer(s) of this feature want an ability to change the DSCR value
of the already running process. I.e. we need some kernel interface which
would accept pid and the register value and change it.
There are 4 options:
1. [not a kernel interface] use ptrace to execute the register changing
command inside the specified pid. The next context switch saves the new
value in the thread_struct. Dirty hack.
2. Add a new syscall which would receive pid + register value and do the
job. A bit too much.
3. Add some hook in /proc filesystem but so far there were no platform
specific bits, right?
4. Implement a static node "/sys/devices/system/cpu/dscr_control". write()
would parse the input stream, call scanf("%d %x", &pid, &dscr) and do the job.
What is the correct approach? Thanks.
--
Alexey
> There are 4 options:
> 1. [not a kernel interface] use ptrace to execute the register changing
> command inside the specified pid. The next context switch saves the new
> value in the thread_struct. Dirty hack.
>
> 2. Add a new syscall which would receive pid + register value and do the
> job. A bit too much.
>
> 3. Add some hook in /proc filesystem but so far there were no platform
> specific bits, right?
>
> 4. Implement a static node "/sys/devices/system/cpu/dscr_control".
> write() would parse the input stream, call scanf("%d %x", &pid, &dscr)
> and do the job.
>
/sys/ interface would be appropriate I believe. But in this way we can take a new
(pid, dscr) and update thread_struct. But there should be a way to enlist all
(pid, dscr) values which are explicitly set by the user and different than that
of /sys/devices/system/cpu/dscr_default. So that we can know which process is holding
to what value of DSCR at any point of time.
> What is the correct approach? Thanks.
>
>
Regards
Anshuman