Re: [PATCH v3 3/9] x86/PVH: permit more physdevop-s to be used by Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.09.2021 16:22, Roger Pau Monné wrote:
> On Tue, Sep 21, 2021 at 09:17:37AM +0200, Jan Beulich wrote:
>> Certain notifications of Dom0 to Xen are independent of the mode Dom0 is
>> running in. Permit further PCI related ones (only their modern forms).
>> Also include the USB2 debug port operation at this occasion.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> I'm uncertain about the has_vpci() part of the check: I would think
>> is_hardware_domain() is both sufficient and concise. Without vPCI a PVH
>> Dom0 won't see any PCI devices in the first place (and hence would
>> effectively be non-functioning). Dropping this would in particular make
>> PHYSDEVOP_dbgp_op better fit in the mix.
>> ---
>> v3: New.
>>
>> --- a/xen/arch/x86/hvm/hypercall.c
>> +++ b/xen/arch/x86/hvm/hypercall.c
>> @@ -94,6 +94,12 @@ static long hvm_physdev_op(int cmd, XEN_
>>          break;
>>  
>>      case PHYSDEVOP_pci_mmcfg_reserved:
>> +    case PHYSDEVOP_pci_device_add:
>> +    case PHYSDEVOP_pci_device_remove:
>> +    case PHYSDEVOP_restore_msi_ext:
> 
> Hm, I'm slightly unsure we need the restore operation. Wouldn't it be
> better to just reset all device state on suspend and then let dom0
> restore it's state as it does on native?

Hmm - Linux (even after my patch separating XEN_DOM0 from XEN_PV)
only issues this call when running in PV mode, so from that
perspective leaving it out would be okay. (Otherwise, i.e. if we
decide to permit its use, I guess we would better also permit
PHYSDEVOP_restore_msi. Somehow I had managed to not spot that.)
However, ...

> Maybe there's some wrinkle that prevents that from working properly.

... Xen might be using MSI for the serial console, and I'm not sure
this interrupt would get properly re-setup.

>> +    case PHYSDEVOP_dbgp_op:
>> +    case PHYSDEVOP_prepare_msix:
>> +    case PHYSDEVOP_release_msix:
> 
> Albeit I think those two operations won't strictly conflict with vPCI
> usage (as they require no MSIX entries to be activ) I still wonder
> whether we will end up needing them on a PVH dom0. They are used by
> pciback and it's not yet clear how we will end up using pciback on a
> PVH dom0, hence I would prefer if we could leave them out until
> strictly required.

Even without a clear plan towards pciback, do you have any idea how
their function could sensibly be replaced in the PVH case? If there
is at least a rough idea, I'd be fine leaving them out here.

Jan