Messages in this thread |  | | | Date | Mon, 25 May 2009 05:51:58 +0200 | | From | Ingo Molnar <> | | Subject | Re: [GIT PULL] Xen APIC hooks (with io_apic_ops) |
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* Avi Kivity <avi@redhat.com> wrote:
> Ingo Molnar wrote: >>> IO APIC operations are not even slightly performance critical? Are >>> they ever used on the interrupt delivery path? >>> >> >> Since they are not performance critical, then why doesnt Xen catch the >> IO-APIC accesses, and virtualizes the device? >> >> If you want to hook into the IO-APIC code at such a low level, why >> dont you hook into the _hardware_ API - i.e. catch those setup/routing >> modifications to the IO-APIC space. No Linux changes are needed in that >> case. >> > > When x2apic is enabled, and EOI broadcast is disabled, then the io > apic does become a hot path - it needs to be written for each > level-triggered interrupt EOI. In this case I might want to > paravirtualize the EOI write to exit only if an interrupt is > pending; otherwise communicate via shared memory. > > We do something similar for Windows (by patching it) very > successfully; Windows likes to touch the APIC TPR ~ 100,000 times > per second, usually without triggering an interrupt. We hijack > these writes, do the checks in guest context, and only exit if the > TPR write would trigger an interrupt.
I suspect you aware of that this is about the io-apic not the local APIC. The local apic methods are already driver-ized - and they sit closer to the CPU so they matter more to performance.
> (kvm will likely gain x2apic support in 2.6.32; patches have > already been posted)
ok. This points in the direction of the io-apic driver abstraction from Jeremy being the right long-term approach. We already have a few quirks that could be cleaned up by using a proper driver interface.
Ingo
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