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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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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