On Mon, Oct 28, 2013 at 08:58:08PM -0600, David Ahern wrote: > On 10/28/13 7:15 AM, Peter Zijlstra wrote: > >>Any suggestions on how to do this and without impacting performance. I > >>noticed the MSR path seems to take about twice as long as the current > >>implementation (which I believe results in rdtsc in the VM for x86 with > >>stable TSC). > > > >So assuming all the TSCs are in fact stable; you could implement this by > >syncing up the guest TSC to the host TSC on guest boot. I don't think > >anything _should_ rely on the absolute TSC value. > > > >Of course you then also need to make sure the host and guest tsc > >multipliers (cyc2ns) are identical, you can play games with > >cyc2ns_offset if you're brave. > > > > This and the method Gleb mentioned both are going to be complex and > fragile -- based assumptions on how the perf_clock timestamps are > generated. For example, 489223e assumes you have the tracepoint > enabled at VM start with some means of capturing the data (e.g., a > perf-session active). We can think of other ways to provide tsc offset to perf. > In both cases the end result requires piecing > together and re-generating the VM's timestamp on the events. For > perf this means either modifying the tool to take parameters and an > algorithm on how to modify the timestamp or a homegrown tool to > regenerate the file with updated timestamps. > > To back out a bit, my end goal is to be able to create and merge > perf-events from any context on a KVM-based host -- guest userspace, > guest kernel space, host userspace and host kernel space (userspace > events with a perf-clock timestamp is another topic ;-)). Having the > events generated with the proper timestamp is the simpler approach > than trying to collect various tidbits of data, massage timestamps > (and hoping the clock source hasn't changed) and then merge events. > So can you explain a little bit more about how this will work? You run perf on a host and get both host and guest events? How do you pass events from guest to host in this case? > And then for the cherry on top a design that works across > architectures (e.g., x86 now, but arm later). > MSR is x86 thing. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
