On 10/29/2012 10:40 PM, Marcelo Tosatti wrote: > 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). > Duplicating is indeed no the end of the world. But one note: * If it is page-size aligned, it is automatically cacheline aligned. Since we have to export the user page *anyway*, this is a non-issue. That said, duplicating instead of integrating, which is technically possible, is a design decision, and it needs to be documented somewhere aside from this mail thread. -- 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
