On 10/29/2012 08:40 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). > What would be the changes required to remove the duplication? If it's just page alignment, then is seems even smaller. In addition we avoid expanding the ABI again. -- error compiling committee.c: too many arguments to function -- 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
