Thanks for reviewing this patch and the rest of the gang. On Oct 6, 2014, at 11:57 PM, Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote: > 2014-10-03 01:10+0300, Nadav Amit: >> Setting the TSC deadline MSR that are initiated by the host (using ioctl's) may >> cause superfluous interrupt. This occurs in the following case: >> >> 1. A TSC deadline timer interrupt is pending. >> 2. TSC deadline was still not cleared (which happens during vcpu_run). >> 3. Userspace uses KVM_GET_MSRS/KVM_SET_MSRS to load the same deadline msr. >> >> To solve this situation, ignore host initiated TSC deadline writes that do not >> change the deadline value. > > I find this change slightly dubious … Why? I see similar handling of MSR_TSC_ADJUST. > - why does the userspace do that? It seems qemu’s kvm_cpu_exec does so when it calls kvm_arch_put_registers. It is pretty much done after every exit to userspace. > - why is host_initiated required? Since if the guest writes to the MSR, it means it wants to rearm the TSC deadline. Even if the deadline passed, interrupt should be triggered. If the guest writes the same value on the deadline MSR twice, it might expect two interrupts. > > Thanks. > > It seems like an performance improvement, so why shouldn't return when > 'data <= tscdeadline && pending()' > or even > 'data <= now() && pending()' > > (Sorry, I ran out of time to search for answers today.) The bug I encountered is not a performance issue, but a superfluous interrupt (functional bug). As I said, the guest may write a new deadline MSR value which is in the past and expect an interrupt. Nadav-- 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
