Re: [GIT PULL] Xen APIC hooks (with io_apic_ops)

Jeremy Fitzhardinge
2009-05-20T17:57:25Z

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