On Wed, Nov 14, 2018 at 10:19 AM Nadav Har'El <n...@scylladb.com> wrote:
> > On Tue, Oct 23, 2018 at 2:47 AM Rick Payne <ri...@rossfell.co.uk> wrote: > *Glauber*, I'd love to get your opinion about this patch. Clearly our > original assumption that we can only call the wallclock MSR once, was > wrong. If I understand correctly, Linux guest does the same thing as this > patch - use the wallclock MSR every time the wall clock is required - so > this patch is the right way to go. But isn't this slow, and perhaps > pointless, to call this MSR every time the wall clock is needed? Was this > the intention when the pv clock ABI was designed? > So, I consulted with Glauber in private, and his opinion was that as I noted later in this thread - Linux actually does *not* do what this patch does (write the wallclock MSR every time the wallclock is read). Glauber said that I was rightly worried that the approach in this patch - doing the MSR on every wall clock read is is too costly (remember that every MSR write requires an exit to the hypervisor) A much more efficient approach could be to run this MSR every once in a while (e.g., once each second). One possibility could be to have a thread (or re-use some existing system thread) which periodically refreshes the wallclock offset. Another possibility is to have the wallclock function check if enough monotonic has passed since the last wallclock offset refresh, and if enough time has passed (remember to check in a thread-safe way!), refresh it. Each of these approaches has ugly sides, so I wonder what Linux does for this. Would be great if you or someone could try to understand what a Linux guest does here. Nadav. -- You received this message because you are subscribed to the Google Groups "OSv Development" group. To unsubscribe from this group and stop receiving emails from it, send an email to osv-dev+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
