On 02/10/2015 11:58, Peter Maydell wrote: > On 2 October 2015 at 10:30, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: >> >> >> On 02/10/2015 09:28, Pavel Fedin wrote: >>> 2. Another possible approach, based on how device tree binding is handled by Linux. It is possible >>> to remove virtual timer IRQ from the device tree, in this case the kernel reverts to using physical >>> timer. When running under hypervisor, accesses to physical CP15 timer are trapped into HYP, >>> therefore we can forward them to userspace using new exit code, something like KVM_EXIT_REG_ACCESS. >>> In this case the timer would be also emulated by the userspace, which is slower, but allows better >>> emulation. Also, this could be used in order to run some other guests which just expect physical >>> timer to be there. >>> >>> Both approaches have their own limitations, but anyway this is much better than nothing. What do >>> you think, and which approach do you like more? >> >> I like the latter. But I guess one could even do both? > > I definitely dislike the latter -- userspace ends up having to > emulate part of the CPU even though that CPU support is really > there in hardware. Also it requires us to edit the device tree, > which means it won't work at all on boards other than 'virt' > where we use the kernel's device tree rather than creating our > own. Better for the kernel to forward the timer > interrupts back out to userspace's irq controller. How do boards other than 'virt' work when emulated without KVM? It must be possible to emulate the physical timer in QEMU. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
