Re: [PATCH] KVM: Add system call KVM_VERIFY_MSI to verify MSI vector

[chenxiang (M)]

Hi Marc,


在 2022/11/10 18:28, Marc Zyngier 写道:
> On Wed, 09 Nov 2022 06:21:18 +0000,
> "chenxiang (M)" <chenxiang66@hisilicon.com> wrote:
> > Hi Marc,
> >
> >
> > 在 2022/11/8 20:47, Marc Zyngier 写道:
> > > On Tue, 08 Nov 2022 08:08:57 +0000,
> > > chenxiang <chenxiang66@hisilicon.com> wrote:
> > > > From: Xiang Chen <chenxiang66@hisilicon.com>
> > > >
> > > > Currently the numbers of MSI vectors come from register PCI_MSI_FLAGS
> > > > which should be power-of-2, but in some scenaries it is not the same as
> > > > the number that driver requires in guest, for example, a PCI driver wants
> > > > to allocate 6 MSI vecotrs in guest, but as the limitation, it will allocate
> > > > 8 MSI vectors. So it requires 8 MSI vectors in qemu while the driver in
> > > > guest only wants to allocate 6 MSI vectors.
> > > >
> > > > When GICv4.1 is enabled, we can see some exception print as following for
> > > > above scenaro:
> > > > vfio-pci 0000:3a:00.1: irq bypass producer (token 000000008f08224d) 
> > > > registration fails:66311
> > > >
> > > > In order to verify whether a MSI vector is valid, add KVM_VERIFY_MSI to do
> > > > that. If there is a mapping, return 0, otherwise return negative value.
> > > >
> > > > This is the kernel part of adding system call KVM_VERIFY_MSI.
> > > Exposing something that is an internal implementation detail to
> > > userspace feels like the absolute wrong way to solve this issue.
> > >
> > > Can you please characterise the issue you're having? Is it that vfio
> > > tries to enable an interrupt for which there is no virtual ITS
> > > mapping? Shouldn't we instead try and manage this in the kernel?
> > Before i reported the issue to community, you gave a suggestion about
> > the issue, but not sure whether i misundertood your meaning.
> > You can refer to the link for more details about the issue.
> > https://lkml.kernel.org/lkml/87cze9lcut.wl-maz@kernel.org/T/
> Right. It would have been helpful to mention this earlier. Anyway, I
> would really like this to be done without involving userspace at all.
>
> But first, can you please confirm that the VM works as expected
> despite the message?
Yes, it works well except the message.

> If that's the case, we only need to handle the
> case where this is a multi-MSI setup, and I think this can be done in
> VFIO, without involving userspace.

It seems we can verify every kvm_msi for multi-MSI setup in function 
vfio_pci_set_msi_trigger().
If it is a invalid MSI vector, then we can decrease the numer of MSI 
vectors before  calling vfio_msi_set_block().

> Thanks,
>
>         M.