On Mon, Oct 29, 2012 at 10:44:41AM -0700, Jeremy Fitzhardinge wrote: > On 10/29/2012 07:45 AM, Glauber Costa wrote: > > On 10/24/2012 05:13 PM, 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> > > Can you please be a bit more specific about why we need this? Why does > > the host need to provide us with two pages with the exact same data? Why > > can't just do it with mapping tricks in the guest? > > In Xen the pvclock structure is embedded within a pile of other stuff > that shouldn't be mapped into guest memory, so providing for a second > location allows it to be placed whereever is convenient for the guest. > That's a restriction of the Xen ABI, but I don't know if it affects KVM. > > J It is possible to share the data for KVM in theory, but: - It is a small amount of memory. - It requires aligning to page size (the in-kernel percpu array is currently cacheline aligned). - It is possible to modify flags separately for userspace/kernelspace, if desired. This justifies the duplication IMO (code is simple and clean). -- 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
