Jeremy Fitzhardinge wrote:
This is a fairly close match to Xen's requirements. Certainly, anything
APIC-related is useless for us, since there's no APIC emulation going on.
I won't speak for Zach, but his counter-argument is generally along the
lines of "we can just make use of the existing code with a couple of
little hooks near the bottom". But I wonder if the existing genapic
interface can be used (or extended) to cover these cases without having
needing to have APIC-level interfaces in paravirt_ops.
Because we faithfully emulate the APIC and IO-APIC, that is the
underlying hardware for us, and we don't have a fancy paravirtualized
interrupt controller because there is no need for it. The only
obstruction to this approach is that trapping and emulating APIC access
is slow. And some APIC registers have side effects on read. So we
simply replace APIC read / write with faster hypercalls.
Of course we can create a bunch of new code to use the genapic
interface. It is just a matter of copying apic.c and io-apic.c verbatim
and applying the sed command s/apic/vmi_apic/g. We can easily do this,
but the only point would be to eliminate the low-level APIC access
paravirt-op, which is not a maintenance burden, performance problem, or
encumberance on anyone. So it would be purely a cleanliness thing.
Doubling code to make two separate copies when the interface in question
is already well abstracted and contained in a header file doesn't make
it cleaner, at least to me.
Zach
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