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Besides the fact that we did not have that for the current supported boards, that makes sense for this new, virtualized mode, that is meant to be run on top of full-fledged x86 64 CPUs. By having xAPIC mode access only, Jailhouse has to intercept those MMIO reads and writes, in order to examine what they do and arbitrate if it's safe or not (e.g. not all values are accepted to ICR register). This means that we can't run away from having a VM-exit event for each and every access to APIC memory region and this impacts the latency the guest OS observes over bare metal a lot. When in x2APIC mode, Jailhouse does not require VM-exits for MSR accesses other that writes to the ICR register, so the latency the guest observes is reduced to almost zero. Here are some outputs of the the command line $ sudo ./tools/jailhouse cell stats tiny-demo on a Jailhouse's root cell console, for one of the Zephyr demos using LOAPIC timers, left for a couple of seconds: Statistics for tiny-demo cell (x2APIC root, x2APIC inmate) COUNTER SUM PER SEC vmexits_total 7 0 vmexits_management 3 0 vmexits_cr 2 0 vmexits_cpuid 1 0 vmexits_msr 1 0 vmexits_exception 0 0 vmexits_hypercall 0 0 vmexits_mmio 0 0 vmexits_pio 0 0 vmexits_xapic 0 0 vmexits_xsetbv 0 0 Statistics for tiny-demo cell (xAPIC root, xAPIC inmate) COUNTER SUM PER SEC vmexits_total 4087 40 vmexits_xapic 4080 40 vmexits_management 3 0 vmexits_cr 2 0 vmexits_cpuid 1 0 vmexits_msr 1 0 vmexits_exception 0 0 vmexits_hypercall 0 0 vmexits_mmio 0 0 vmexits_pio 0 0 vmexits_xsetbv 0 0 Statistics for tiny-demo cell (xAPIC root, x2APIC inmate) COUNTER SUM PER SEC vmexits_total 4087 40 vmexits_msr 4080 40 vmexits_management 3 0 vmexits_cr 2 0 vmexits_cpuid 1 0 vmexits_exception 0 0 vmexits_hypercall 0 0 vmexits_mmio 0 0 vmexits_pio 0 0 vmexits_xapic 0 0 vmexits_xsetbv 0 0 See that under x2APIC mode on both Jailhouse/root-cell and guest, the interruptions from the hypervisor are minimal. That is not the case when Jailhouse is on xAPIC mode, though. Note also that, as a plus, x2APIC accesses on the guest will map to xAPIC MMIO on the hypervisor just fine. Signed-off-by: Gustavo Lima Chaves <gustavo.lima.chaves@intel.com> |
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| .. | ||
| bluetooth | ||
| clock_control | ||
| console | ||
| gpio | ||
| ieee802154 | ||
| pci | ||
| spi | ||
| usb | ||
| ioapic.h | ||
| loapic.h | ||
| mvic.h | ||
| sysapic.h | ||
| system_timer.h | ||