----- Original Message ----- > From: "Marcelo Tosatti" <mtosatti@xxxxxxxxxx> > To: "Paolo Bonzini" <pbonzini@xxxxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, "Radim Krčmář" <rkrcmar@xxxxxxxxxx> > Sent: Wednesday, November 9, 2016 9:17:19 PM > Subject: Re: [PATCH] kvm: kvmclock: let KVM_GET_CLOCK return whether the master clock is in use > > On Wed, Nov 09, 2016 at 06:12:50PM -0200, Marcelo Tosatti wrote: > > On Wed, Nov 09, 2016 at 05:48:15PM +0100, Paolo Bonzini wrote: > > > Userspace can read the exact value of kvmclock by reading the TSC > > > and fetching the timekeeping parameters out of guest memory. This > > > however is brittle and not necessary anymore with KVM 4.11. > > > > Hi Paolo, > > > > Can you point to commit or explanation why that is not the case anymore? > > Thanks > > I don't see how thats possible given the: > > * TSC > * TSC (timer interrupt + TSC deltas) (AKA host TSC clocksource and > * CLOCK_MONOTONIC). > > Clocks currently drift from each other (therefore are not monotonic). __get_kernel_ns now computes the guest's kvmclock directly when using the master clock. Instead of peeking at guest memory, it does it from the hypervisor's copy of the parameters. Paolo -- 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
