On 11/4/25 6:41 PM, Jason Gunthorpe wrote:
> On Tue, Nov 04, 2025 at 09:11:55AM -0800, Nicolin Chen wrote:
>> On Tue, Nov 04, 2025 at 11:35:35AM -0400, Jason Gunthorpe wrote:
>>> On Tue, Nov 04, 2025 at 03:20:59PM +0000, Shameer Kolothum wrote:
>>>>> On Tue, Nov 04, 2025 at 02:58:44PM +0000, Shameer Kolothum wrote:
>>>>>>> Sure it is trapped, but nothing should be looking at the MSI address
>>>>>>> from the guest, it is meaningless and wrong information. Just ignore
>>>>>>> it.
>>>>>> Hmm.. we need to setup the doorbell address correctly.
>>>>>> If we don't do the translation here, it will use the Guest IOVA
>>>>>> address. Remember, we are using the IORT RMR identity mapping to get
>>>>>> MSI working.
>>>>> Either you use the RMR value, which is forced by the kernel into the
>>>>> physical MSI through iommufd and kernel ignores anything qemu
>>>>> does. So fully ignore the guest's vMSI address.
>>>> Well, we are sort of trying to do the same through this patch here.
>>>> But to avoid a "translation" completely it will involve some changes to
>>>> Qemu pci subsystem. I think this is the least intrusive path I can think
>>>> of now. And this is a one time setup mostly.
>>> Should be explained in the commit message that the translation is
>>> pointless. I'm not sure about this, any translation seems risky
>>> because it could fail. The guest can use any IOVA for MSI and none may
>>> fail.
in general the translation is not pointless (I mean when RMR are not
applied). In case a vhost device (virtio-net) for instance is protected
by SMMU, vhost triggers irqfds upon which a gsi is injected in vgic.
This latter does irq_routing mapping and this gsi is associated to an
MSI address/data. If the MSI address is wrong, ie. not corresponding to
the vITS gpa doorbell, kernel kvm/vgic/vgic-its.c vgic_its_trigger_msi
will fail to inject the MSI on guest since
vgic_msi_to_its/__vgic_doorbell_to_its will fail to find the ITS
instance to inject in.
Thanks
Eric
>> In the current design of KVM in QEMU, it does a generic translation
>> from gIOVA->gPA for the doorbell location to inject IRQ, whether VM
>> has an accelerated IOMMU or an emulated IOMMU.
> And what happens if the translation fails because there is no mapping?
> It should be ignored for this case and not ignored for others.
>
> Jason
>
