Re: [Xen-devel] [PATCH V2] vm_event: Allow subscribing to write events for specific MSR-s

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

>> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
>> index 2457698..875c09a 100644
>> --- a/xen/include/public/domctl.h
>> +++ b/xen/include/public/domctl.h
>> @@ -1107,8 +1107,7 @@ struct xen_domctl_monitor_op {
>>          } mov_to_cr;
>>
>>          struct {
>> -            /* Enable the capture of an extended set of MSRs */
>> -            uint8_t extended_capture;
>> +            uint32_t msr;
>
> Whoa there. Isn't it expanding the structure? Will this be backwards
> compatible? What if somebody is using an older version of xen-access
> against this hypervisor? Will they work?
>
> Perhaps this should have a new struct / sub-ops? And the old
> 'mov_to_msr' will just re-use this new fangled code?

In addition to Andrew's comments, I think simply changing
VM_EVENT_INTERFACE_VERSION should be enough for xen-access-like clients
to figure out the incompatibility.

This is an independent system from VM_EVENT, so IMHO the two shouldn't be mixed. The union size right now is 24-bits so if a uint16_t is enough for the bitmask that should be used instead. That way we don't end up growing the struct size.

Tamas

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