Hi Jintack, On Wed, Feb 01, 2017 at 12:43:00PM -0500, 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. > > This patch series 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 patch series. This patch series will make it easy to add the EL2 > timer support in the future, though. > > Note that Linux VMs booting in EL1 will be unaffected by this patch series > and will continue to use only the virtual timer and this patch series will > therefore not introduce any performance degredation as a result of > trap-and-emulate. > > v2 => v3: > - Rebase on kvmarm/queue > - Take kvm->lock to synchronize cntvoff across all vtimers > - Remove unnecessary function parameters > - Add comments I just gave v3 a test run on my TC2 (32-bit platform) and my guest quickly locks up trying to run cyclictest or when booting the machine it stalls with RCU timeouts. Could you have a look? Thanks, -Christoffer
