Re: [PATCH v8 4/7] xen: Add vmware_port support

[Don Slutz <dslutz@verizon.com>]

On 01/26/15 10:58, Don Slutz wrote:
> On 01/22/15 03:32, Jan Beulich wrote:
>>>>> On 21.01.15 at 18:52, <dslutz@verizon.com> wrote:
>>> On 01/16/15 05:09, Jan Beulich wrote:
>>>>>>> On 03.10.14 at 00:40, <dslutz@verizon.com> wrote:
>>>>> This is a new domain_create() flag, DOMCRF_vmware_port.  It is
>>>>> passed to domctl as XEN_DOMCTL_CDF_vmware_port.
>>>> Can you explain why a HVM param isn't suitable here?
>>>>
>>> The issue is that you need this flag during construct_vmcb() and
>>> construct_vmcs().  While Intel has vmx_update_exception_bitmap()
>>> AMD does not.  So when HVM param's are setup and/or changed there
>>> currently is no way to adjust AMD's exception bitmap.
>>>
>>> So this is the simpler way.
>> But the less desirable one from a design/consistency perspective.
>> Unless other maintainers disagree, I'd like to see this changed.
> 
> Ok, but will wait some time to see if "Unless other maintainers disagree"
> 

While coding this is up I have hit issues that I need input on:

As a HVM_PARAM_ item, I would assume I should be following
what HVM_PARAM_VIRIDIAN does.  It has this comment:

            case HVM_PARAM_VIRIDIAN:
                /* This should only ever be set once by the tools and
read by the guest. */

Which is almost true.  However the code allows you to change from 0 to
non-zero any time in the life of the DomU.  I am assuming that this is
why xc_domain_save() and xc_domain_restore() save and restore this
HVM_PARAM_ item.

With the enable of vmware_port the same way, I feel it would be a bug
to allow the enable after "create" to not also adjust QEMU.  Currently
there is no way for the hypervisor to tell QEMU to enable vmware_port
handling.  So to avoid adding this code to xen and QEMU, it looks to
me that adding code to make this a true write only 1 time would be
needed so that you cannot use the hyper call to change later.

So, should I extend this change to cover other HVM_PARAM_?

Is all this additional code (xc_domain_save(), xc_domain_restore(),
write only 1 time) still better then a domain_create() flag?

   -Don Slutz