Hi Marc, On Sun, Jan 29, 2017 at 10:55 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > Hi Jintack, > > On Fri, Jan 27 2017 at 01:04:50 AM, Jintack Lim <jintack@xxxxxxxxxxxxxxx> 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. > > Thanks for respining this series. Overall, this looks quite good, and > the couple of comments I have should be easy to address. Thanks for the review! > > My main concern is that we do expose a timer that doesn't hide > CNTVOFF. I appreciate that that was already the case, since CNTPCT was > always available (and this avoided trapping the counter), but maybe we > should have a way for userspace to ask for a mode where CNTPCT=CNTVCT, > byt trapping the physical counter and taking CNTVOFF in all physical > timer calculations. As discussed in the other thread, I think we can expose CNTVOFF to the guest OS. I have a patch that lets the guest hypervisor observe CNTVCT = CNTPCT - offset (virtual CNTVOFF_EL2) and I will include it in the next nesting patch series. Thanks, Jintack > > I'm pretty sure you've addressed this one way or another in your nested > virt series, so maybe extracting the relevant patches and adding them on > top of this series could be an option? > > Thanks, > > M. > -- > Jazz is not dead. It just smells funny. >
