On Thu, Feb 2, 2017 at 7:31 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote: > 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. Ok. It's my fault not to specify that the emulated physical timer is supported/tested on arm64. On 32-bit platform, it is supposed to show the same behavior as before, but I haven't tested. Were you using the physical timer or the virtual timer for the guest? > > Could you have a look? Sure, I'll have a look. I don't have access to my Cubietruck today, but I can work on that tomorrow. > > Thanks, > -Christoffer >
