On Fri, Nov 02, 2012 at 02:23:06PM +0400, Glauber Costa wrote: > On 11/02/2012 01:39 AM, Marcelo Tosatti wrote: > > On Thu, Nov 01, 2012 at 06:28:31PM +0400, Glauber Costa wrote: > >> On 11/01/2012 02:47 AM, Marcelo Tosatti wrote: > >>> Allow a guest to register a second location for the VCPU time info > >>> > >>> structure for each vcpu (as described by MSR_KVM_SYSTEM_TIME_NEW). > >>> This is intended to allow the guest kernel to map this information > >>> into a usermode accessible page, so that usermode can efficiently > >>> calculate system time from the TSC without having to make a syscall. > >>> > >>> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > >>> > >> > >> Changelog doesn't make a lot of sense. (specially from first line to the > >> second). Please add in here the reasons why we can't (or decided not to) > >> use the same page. The info in the last mail thread is good enough, just > >> put it here. > > > > Fixed. > > > >>> Index: vsyscall/arch/x86/include/asm/kvm_para.h > >>> =================================================================== > >>> --- vsyscall.orig/arch/x86/include/asm/kvm_para.h > >>> +++ vsyscall/arch/x86/include/asm/kvm_para.h > >>> @@ -23,6 +23,7 @@ > >>> #define KVM_FEATURE_ASYNC_PF 4 > >>> #define KVM_FEATURE_STEAL_TIME 5 > >>> #define KVM_FEATURE_PV_EOI 6 > >>> +#define KVM_FEATURE_USERSPACE_CLOCKSOURCE 7 > >>> > >>> /* The last 8 bits are used to indicate how to interpret the flags field > >>> * in pvclock structure. If no bits are set, all flags are ignored. > >>> @@ -39,6 +40,7 @@ > >>> #define MSR_KVM_ASYNC_PF_EN 0x4b564d02 > >>> #define MSR_KVM_STEAL_TIME 0x4b564d03 > >>> #define MSR_KVM_PV_EOI_EN 0x4b564d04 > >>> +#define MSR_KVM_USERSPACE_TIME 0x4b564d05 > >>> > >> > >> I accept that it is possible that we may be better off with the page > >> mapped twice. > >> > >> But why do we need an extra MSR? When, and why, would you enable the > >> kernel-based pvclock, but disabled the userspace pvclock? > > > > Because there is no stable TSC available, for example (which cannot > > be used to measure passage of time). > > > > What you say is true, but completely unrelated. I am not talking about > situations in which userspace pvclock is available and you end up not > using it. > > I am talking about situations in which it is available, you are capable > of using it, but then decides for some reason to permanently disabled - > as in not setting it up altogether. > > It seems to me that if the host has code to deal with userspace pvclock, > and you already coded the guest in a way that you may or may not use it > (dependent on the value of the stable bit), you could very well only > check for the cpuid flag, and do the guest setup if available - skipping > this MSR dance altogether. > > Now, of course, there is the problem of communicating the address in > which the guest expects the page to be. Skipping the MSR setup would > require it to be more or less at a fixed location. We could in principle > lay them down together with the already existing pvclock structure. (But > granted, I am not sure it is worth it...) > > I think in general, this question deserves a bit more of attention. We > are about to have just the perfect opportunity for this next week, so > let's use it. In essence you are proposing a different interface to communicate the "userspace vsyscall pvclock area", other than the MSR, right? If so: 1. What is the problem with the MSR interface. 2. What advantage this new interface (which honestly i do not understand) provides? -- 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
