On 26/12/16 17:11, Jintack Lim wrote: > The ARM architecture defines the EL1 physical timer and the virtual > timer, and it is reasonable for an OS to expect to be able to access > both. However, the current KVM implementation does not provide the EL1 > physical timer to VMs but terminates VMs on access to the timer. > > On VHE systems, this would be as simple as allowing full access to the > EL1 physical timer to VMs because the KVM host does not use the EL1 > physical timer. However, on non-VHE systems, the KVM host already uses > the EL1 physical timer which prevents us from granting full access of > the EL1 physical timer to VMs. > > This patchset enables VMs to use the EL1 physical timer through > trap-and-emulate. The KVM host emulates each EL1 physical timer > register access and sets up the background timer accordingly. When the > background timer expires, the KVM host injects EL1 physical timer > interrupts to the VM. Alternatively, it's also possible to allow VMs to > access the EL1 physical timer without trapping. However, this requires > somehow using the EL2 physical timer for the Linux host while running > the VM instead of the EL1 physical timer. Right now I just implemented > trap-and-emulate because this was straightforward to do, and I leave it > to future work to determine if transferring the EL1 physical timer state > to the EL2 timer provides any performance benefit. > > This feature will be useful for any OS that wishes to access the EL1 > physical timer. Nested virtualization is one of those use cases. A > nested hypervisor running inside a VM would think it has full access to > the hardware and naturally tries to use the EL1 physical timer as Linux > would do. Other nested hypervisors may try to use the EL2 physical timer > as Xen would do, but supporting the EL2 physical timer to the VM is out > of scope of this patchset. This patchset will make it easy to add the > EL2 timer support in the future, though. > > Note, Linux VMs booting in EL1 will be unaffected by this patch set and > will continue to use only the virtual timer and this patch set will > therefore not introduce any performance degredation as a result of > trap-and-emulate. Hi Jintack, Any chance you could address Christoffer's comments and respin this series? This looks like a good enhancement to our emulation, and definitely a requirement for the nested work, so I'm obviously keen on it. Thanks, M. -- Jazz is not dead. It just smells funny... -- 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
