Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 2 Jul 2015 12:26:04 -0700 | Subject | Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr |
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On Thu, Jul 2, 2015 at 12:22 PM, H. Peter Anvin <hpa@zytor.com> wrote: > On 07/01/2015 09:38 AM, Brown, Len wrote: >> >> BTW. I've had a discussion w/ LLNL about their needs, >> both for security and performance. For security, as concluded >> by this thread, a white list is the only way to go. >> I'm thinking a bit-vector of allowed MSR offsets... >> For performance, they absolutely can not afford a system call >> for every single MSR access. Here an ioctl to have the >> msr driver perform a vector of accesses in a single system >> call seems the way to go. I can prototype both of these >> using turbostat as the customer. >> > > Every time I have heard about people having issues with performance for > MSR access, it is because they are doing cross-CPU accesses which means > a neverending stream of IPIs. You get immensely better performance by > tying a thread to a CPU and only accessing the local CPU from that > thread. This has addressed any performance problems anyone has ever > come to me with. As Andy and Ingo have already pointed out, the MSR > access itself is pretty much as expensive as the system call overhead.
To be fair, before we had opportunistic sysret, CONFIG_CONTEXT_TRACKING was *extremely* expensive. Even now, it's still pretty bad.
Len, do you know what configuration and kernel version this was on or what the apparent syscall overhead was?
--Andy
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