Re: [PATCH 2/2][RFC] KVM: Emulate MSI-X table and PBA in kernel

Michael S. Tsirkin
2011-01-02T11:53:10Z

On Sun, Jan 02, 2011 at 12:58:50PM +0200, Avi Kivity wrote:
> On 01/02/2011 12:39 PM, Michael S. Tsirkin wrote:
> >> >
> >> >I agree. At least it's not a regression. And in fact we haven't seen any device
> >> >driver use this. I've checked Linux kernel code, found no one used PCI_MSIX_PBA or
> >> >msix_pba_offset_reg().
> >> >
> >> >I guess it's fine to get MSI-X mask part in first, then deal with PBA part if
> >> >necessary - though we haven't seen any driver use it so far. It won't be worse
> >> >with this patch anyway...
> >>
> >> In a way it is worse because before, the fix would belong in user
> >> space, which is easier to test and distribute. Now we have to fix
> >> it in the kernel.
> >>
> >> However I recognize that drivers which rely on the pending bit are
> >> rare/nonexistent (likely on in preboot environments where interrupts
> >> are hard), so even if we do code it, it will likely be incorrect
> >> (certainly without a test).
> >>
> >> So I'll accept the patch without PBA. Michael, what about
> >> supporting virtio? Can we base something on this patch?
> >
> >I don't see how userspace can send interrupts with this
> >interface unfortunately. We also need irqfd support ...
>
> Sure we'll need additions to that interface.

What I suggested is
1. an ioctl to map phy address + size to table id
2. a new gsi type with a table id + entry number.

If we have that, assigned devices, virtio and vhost-net can work
mostly as is, with just the mask bits accelerated.

> What about vhost-net and vfio? I thought that they could emulate
> the mask bits:
>
> - KVM_MMIOFD(vmfd, mmio_range, fd1, fd2) associates an mmio range with an fd
> - writel(mmio_range) or readl(mmio_range) from the guest causes a
> command to be written to fd1
> - for readl(), read from fd2 to see the result (works nicely for
> "pci read flushes posted writes")
>
> this allows interesting stuff to be implemented in separate
> processes, threads, or kernel modules.

This could work. Some thought needs to be given to how we make sure that
an appropriate type of file is passed in. Maybe using a netlink
based connector for this a good idea?

OTOH if we have MSIX mask bit emulation in kvm anyway, using it makes
sense ...

> --
> error compiling committee.c: too many arguments to function