Messages in this thread | | | Date | Wed, 20 May 2009 10:57:18 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [GIT PULL] Xen APIC hooks (with io_apic_ops) |
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Ingo Molnar wrote: > 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. >
Yes, these changes aren't for a performance reason. It's a case where a few lines change in Linux saves many hundreds or thousands of lines change in Xen.
Xen doesn't have an internal mechanism for emulating devices via pagefaults (that's generally handled by a qemu instance running as part of a guest domain), so there's no mechanism to map and emulate the io-apic. Putting such support into Xen would mean adding a pile of new infrastructure to support this case.
Unlike the mtrr discussion, where the msr read/write ops would allow us to emulate the mtrr within the Xen-specific parts of the kernel, the io-apic ops are just accessed via normal memory writes which we can't hook, so it would have to be done within Xen.
The other thing I thought about was putting a hook in the Linux pagefault handler, so we could emulate the ioapic at that level. But putting a hook in a very hot path to avoid code changes in a cold path doesn't make any sense. (Same applies to doing PF emulation within Xen; that's an even hotter path than Linux's.)
J
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