Re: [Xen-devel] [PATCH RFC v1 33/74] x86/guest: enable event channels upcalls

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/01/18 15:07, Jan Beulich wrote:
>>>> On 04.01.18 at 14:05, <wei.liu2@xxxxxxxxxx> wrote:
>> @@ -30,6 +31,7 @@
>>  bool xen_guest;
>>  
>>  static uint32_t xen_cpuid_base;
>> +static uint8_t evtchn_upcall_vector;
> There being a single global vector, why do you use
> HVMOP_set_evtchn_upcall_vector instead of setting
> HVM_PARAM_CALLBACK_IRQ?

Because another discovery is that HVM_PARAM_CALLBACK_IRQ is subtly
broken.  It is incompatible with L0 Xen choosing to use hardware APIC
assistance, due to its deliberate (ab)use of the IRR state model.

OTOH, there are patches (perhaps later, perhaps not posted yet) which do
try to make use of CALLBACK_IRQ for compatibility on older L0 hypervisors.

~Andrew

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