Question on guest enable msi fail when using GICv4/4.1

[Shaokun Zhang]

Hi Marc,

Thanks for your quick reply.

On 2021/5/7 17:03, Marc Zyngier wrote:
> On Fri, 07 May 2021 06:57:04 +0100,
> Shaokun Zhang <zhangshaokun at hisilicon.com> wrote:
>>
>> [This letter comes from Nianyao Tang]
>>
>> Hi,
>>
>> Using GICv4/4.1 and msi capability, guest vf driver requires 3
>> vectors and enable msi, will lead to guest stuck.
> 
> Stuck how?

Guest serial does not response anymore and guest network shutdown.

> 
>> Qemu gets number of interrupts from Multiple Message Capable field
>> set by guest. This field is aligned to a power of 2(if a function
>> requires 3 vectors, it initializes it to 2).
> 
> So I guess this is a MultiMSI device with 4 vectors, right?
> 

Yes, it can support maximum of 32 msi interrupts, and vf driver only use 3 msi.

>> However, guest driver just sends 3 mapi-cmd to vits and 3 ite
>> entries is recorded in host.  Vfio initializes msi interrupts using
>> the number of interrupts 4 provide by qemu.  When it comes to the
>> 4th msi without ite in vits, in irq_bypass_register_producer,
>> producer and consumer will __connect fail, due to find_ite fail, and
>> do not resume guest.
> 
> Let me rephrase this to check that I understand it:
> - The device has 4 vectors
> - The guest only create mappings for 3 of them
> - VFIO calls kvm_vgic_v4_set_forwarding() for each vector
> - KVM doesn't have a mapping for the 4th vector and returns an error
> - VFIO disable this 4th vector
> 
> Is that correct? If yes, I don't understand why that impacts the guest
> at all. From what I can see, vfio_msi_set_vector_signal() just prints
> a message on the console and carries on.
> 

function calls:
--> vfio_msi_set_vector_signal
   --> irq_bypass_register_producer
      -->__connect

in __connect, add_producer finally calls kvm_vgic_v4_set_forwarding and fails to
get the 4th mapping. When add_producer fail, it does not call cons->start, calls
kvm_arch_irq_bypass_start and then kvm_arm_resume_guest.

Thanks,
Shaokun

>> Do we support this case, Guest function using msi interrupts number
>> not aligned to a power of 2?  Or qemu should provide correct msi
>> interrupts number?
> 
> QEMU cannot know how many vectors are in use, and the guest is free to
> issue mappings for the exact number of vectors it wants to service.
> 
> Please describe what breaks the guest here.
> 
> Thanks,
> 
> 	M.
>