Re: [RFC XEN PATCH 3/6] x86/pvh: shouldn't check pirq flag when map pirq in PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2023 11:09, Huang Rui wrote:
> On Wed, Mar 15, 2023 at 11:57:45PM +0800, Roger Pau Monné wrote:
>> On Sun, Mar 12, 2023 at 03:54:52PM +0800, Huang Rui wrote:
>>> From: Chen Jiqian <Jiqian.Chen@xxxxxxx>
>>>
>>> PVH is also hvm type domain, but PVH hasn't X86_EMU_USE_PIRQ
>>> flag. So, when dom0 is PVH and call PHYSDEVOP_map_pirq, it
>>> will fail at check has_pirq();
>>>
>>> Signed-off-by: Chen Jiqian <Jiqian.Chen@xxxxxxx>
>>> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
>>> ---
>>>  xen/arch/x86/hvm/hypercall.c | 2 --
>>>  1 file changed, 2 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/hvm/hypercall.c b/xen/arch/x86/hvm/hypercall.c
>>> index 405d0a95af..16a2f5c0b3 100644
>>> --- a/xen/arch/x86/hvm/hypercall.c
>>> +++ b/xen/arch/x86/hvm/hypercall.c
>>> @@ -89,8 +89,6 @@ long hvm_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>>> arg)
>>>      case PHYSDEVOP_eoi:
>>>      case PHYSDEVOP_irq_status_query:
>>>      case PHYSDEVOP_get_free_pirq:
>>> -        if ( !has_pirq(currd) )
>>> -            return -ENOSYS;
>>
>> Since I've taken a look at the Linux side of this, it seems like you
>> need PHYSDEVOP_map_pirq and PHYSDEVOP_setup_gsi, but the later is not
>> in this list because has never been available to HVM type guests.
> 
> Do you mean HVM guest only support MSI(-X)?

I don't think that was meant. Instead, as per discussion elsewhere, we
may need to make PHYSDEVOP_setup_gsi available to PVH Dom0. (DomU-s
wouldn't be allowed to use this sub-op, so the statement Roger made
simply doesn't apply to "HVM guest".)

>> I would like to better understand the usage by PVH dom0 for GSI
>> passthrough before deciding on what to do here.  IIRC QEMU also uses
>> PHYSDEVOP_{un,}map_pirq in order to allocate MSI(-X) interrupts.
>>
> 
> The MSI(-X) interrupt doesn't work even on the passthrough device at domU
> even the dom0 is PV domain. It seems a common problem, I remember Christian
> encountered the similar issue as well. So we fallback to use the GSI
> interrupt instead.

Looks like this wants figuring out properly as well then. MSI(-X)
generally works for pass-through devices, from all I know.

Jan