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

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

Hi Roger,

> On 13 Oct 2021, at 09:30, Roger Pau Monné <roger.pau@citrix.com> wrote:
> 
> On Tue, Oct 12, 2021 at 12:38:35PM +0200, Michal Orzel wrote:
>> Hi Roger,
>> 
>> On 11.10.2021 11:27, 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?
>>> 
>>> Thanks, Roger.
>>> 
>> 
>> I have one question about this fix.
>> If I set XEN_DOMCTL_CDF_vpci for dom0 pvh in create_dom0, then in
>> sanitise_domain_config or arch_sanitise_domain_config I have no
>> knowledge on whether I am dom0 or not. I can check if I'm PVH but not if dom0.
>> This would be needed to add a warning if this flag is set but we are not dom0 pvh.
>> 
>> Any ideas?
> 
> I've just realized this is more wrong that I thought. vPCI is
> signaled on x86 in xen_arch_domainconfig.emulation_flags, so
> introducing a top level option for it without removing the arch
> specific one is wrong, as then on x86 we have a duplicated option.
> 
> Then I'm also not sure whether we want to move it from
> emulation_flags, it seems like the more natural place for it to live
> on x86.
> 
> If we really want to make vPCI a CDF option we must deal with the
> removal of XEN_X86_EMU_VPCI, or else you could introduce an arch
> specific flag for vPCI on Arm.

First issue that we have here is that there is no emulation_flags right now on arm.

So I think there are 2 solutions:

- introduce PHYSCAP. The problem here is that it is not a physical capacity and
I think that will hit us in the future for example if we want to use vpci for VIRTIO
even if there is not physical PCI on the platform.

- introduce an emulation flag on arm. The problem here is that there is no emulation
flag right now on arm so adding this feature will require a change of interface in
arch-arm.h and in xen domctl interface. But if we introduce one on Arm, it would allow
the tools to check if CDF_vpci can be set or not which would solve current issues.

I think the second solution is the right one but it cannot be done so near from the
feature freeze.

The CDF flag as introduced right now is not creating any issue and will be used once
the emulation flag will be introduce. We will be able at this stage to set this properly
also on x86 (for dom0 pvh).
Moreover keeping it will allow to continue to merge the remaining part of the PCI
passthrough which are otherwise not possible to be done as they are dependent on this flag.

Can we agree on keep the DOMCTL_CDF_vpci flag and introduce the emulation
flag on Arm after 4.16 release ?

Cheers
Bertrand