On 03/02/17 16:51, Jintack Lim wrote: > 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. That's fine, we can add these later (and maybe I'll just do it, since it is pretty trivial). Thanks, M. -- Jazz is not dead. It just smells funny...
