Re: [PATCH v5 07/11] xen/domctl: Introduce XEN_DOMCTL_CDF_vpci flag

[Michal Orzel <michal.orzel@xxxxxxx>]

On 13.10.2021 08:18, Jan Beulich wrote:
> On 12.10.2021 23:48, Stefano Stabellini wrote:
>> On Mon, 11 Oct 2021, Roger Pau Monné wrote:
>>> On Wed, Oct 06, 2021 at 06:40:33PM +0100, Rahul Singh wrote:
>>>> Introduce XEN_DOMCTL_CDF_vpci flag to enable VPCI support in XEN.
>>>> Reject the use of this new flag for x86 as VPCI is not supported for
>>>> DOMU guests for x86.
>>>
>>> I don't like this approach, XEN_DOMCTL_CDF_vpci should be set for x86
>>> PVH dom0, like we do for any other CDF flags when Xen builds dom0.
>>>
>>> Things like PVH vs PV get translated into CDF flags by create_dom0,
>>> and processed normally by the sanitise_domain_config logic, vPCI
>>> should be handled that way.
>>>
>>> Do you think you could see about fixing this?
>>
>> Andrew suggested to use XEN_SYSCTL_PHYSCAP_vpci to check whether we can
>> set XEN_DOMCTL_CDF_vpci in libxl and it looks like we have consensus on
>> this approach. [1][2]
>>
>> So it makes sense that XEN_DOMCTL_CDF_vpci is only set when
>> XEN_SYSCTL_PHYSCAP_vpci is also set, i.e. XEN_SYSCTL_PHYSCAP_vpci ==
>> XEN_DOMCTL_CDF_vpci.
>>
>> From [2], XEN_SYSCTL_PHYSCAP_vpci is not going to be set on x86, so then
>> XEN_DOMCTL_CDF_vpci should also be left unset?
>>
>> If you think XEN_DOMCTL_CDF_vpci should be set for x86 PVH Dom0, then
>> XEN_SYSCTL_PHYSCAP_vpci should also be set for x86 PVH Dom0.
> 
> Except that XEN_SYSCTL_PHYSCAP_vpci is not a domain specific attribute,
> but a host-wide one.
> 
> Jan
> 
> 
I already prepared a patch introducing XEN_SYSCTL_PHYSCAP_vpci. We agreed
that the cap_vpci should not be set neither for x86 nor ARM. This means that
the flag vpci_is_available, which tells us about vPCI platform support (and is 
used
in condition to set cap_vpci) is set to false by default. Later on it should be 
set by vPCI driver.
For me it does not make sense for XEN_SYSCTL_PHYSCAP_vpci saying that platform 
does not support vPCI
but setting XEN_DOMCTL_CDF_vpci for dom0 pvh.
I would prefer not setting XEN_DOMCTL_CDF_vpci for now at all. This way we have 
a chance
to merge Rahul's series until friday.

Cheers,
Michal