On Thu, Jul 2, 2015 at 12:22 PM, H. Peter Anvin <h...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/