Re: [Xen-devel] [PATCH 2/5] x86/vioapic: allow the vIO APIC to have a variable number of pins

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Fri, Mar 03, 2017 at 04:56:20AM -0700, Jan Beulich wrote:
> >>> On 23.02.17 at 12:52, <roger.pau@xxxxxxxxxx> wrote:
> > @@ -424,40 +424,141 @@ void vioapic_update_EOI(struct domain *d, u8 vector)
> >      spin_unlock(&d->arch.hvm_domain.irq_lock);
> >  }
> >  
> > -static int ioapic_save(struct domain *d, hvm_domain_context_t *h)
> > +#define VIOAPIC_SAVE_CONST offsetof(struct hvm_hw_vioapic, redirtbl)
> 
> What is this redirtbl field, which oddly enough is a pointer (not allowed
> in the public interface) good for? I can't seem to find any use of it
> other than as marker (for use with offsetof()). With all the
> complications here and below plus the fixed number of entries
> remaining to be fixed for actual migration purposes I wonder if you
> wouldn't be better off by keeping the structure mostly as is, simply
> converting the redirtbl[] field to a union (guarded by a __XEN__
> conditional) containing both a fixed size array and a variable size
> one (or to be precise, a zero size one, as a variable size one is not
> allowed there).
> 
> Another alternative would be to introduce a Xen internal sibling
> struct with a variable size array, and with all fields properly build-
> time-asserted to be identical between both.
> 
> I'll skip the rest of the patch assuming much of it could be dropped
> this way.

That was indeed my first approach, but later patches introduce an array of
hvm_hw_vioapic structs (in order to support multiple IO APICs per domain), and
this doesn't work if the struct itself contains a variable size array. I can
encapsulate this inside of a hvm_vioapic struct, like:

struct hvm_vioapic {
    struct hvm_hw_ioapic *vioapic;
}

And make and array of hvm_vioapics instead, but that seemed more cumbersome.

I know this is all quite painful, but I wasn't able to sport a better solution.

Thanks, Roger.

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