On Fri, Feb 3, 2017 at 11:14 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote: > On 03/02/17 15:19, 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 only on arm64. 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. >> >> v3 => v4: >> - Fix a bug that prevents a VM from booting on 32-bit architecture >> - Clarify that the emulated physical timer is only supported on arm64 >> in the cover letter > > Hi Jintack, > > I've now applied this to queue, and will push it out later today. Thanks, Marc. > > Out of curiosity, is there any reason why this is arm64 only? It was simply because I didn't have a convenient 32bit architecture develop environment at hand and didn't spend time to set it up myself :( (As specified in the nesting RFC patch series cover letter, the nesting patches are compiled, but not tested on 32-bit architecture yet.) I guess it's time to set it up. > As far as > I can tell, we're only missing the cp15 handling (both for arm and in > the 32bit handling in arm64). I think so, too. I can't promise when, but I'll try to add those once I set the develop environment. > > Thanks, > > M. > -- > Jazz is not dead. It just smells funny... >
