On Tue, 2012-08-14 at 16:10 +0200, Jan Kiszka wrote:
> On 2012-08-14 16:05, Alex Williamson wrote:
> > On Tue, 2012-08-14 at 15:48 +0200, Jan Kiszka wrote:
> >> Hi Alex,
> >>
> >> you once wrote this comment in device-assignment.c, msix_mmio_write:
> >>
> >> if (!msix_masked(&orig) && msix_masked(entry)) {
> >> /*
> >> * Vector masked, disable it
> >> *
> >> * XXX It's not clear if we can or should actually attempt
> >> * to mask or disable the interrupt. KVM doesn't have
> >> * support for pending bits and kvm_assign_set_msix_entry
> >> * doesn't modify the device hardware mask. Interrupts
> >> * while masked are simply not injected to the guest, so
> >> * are lost. Can we get away with always injecting an
> >> * interrupt on unmask?
> >> */
> >>
> >> I'm wondering what made you think that we won't inject if the vector is
> >> masked like this (ie. in the shadow MSI-X table). Can you recall the
> >> details?
> >>
> >> I'm trying to refactor this code to make the KVM interface a bit more
> >> encapsulating the kernel interface details, not fixing anything. Still,
> >> I would also like to avoid introducing regressions.
> >
> > Yeah, I didn't leave a very good comment there. I'm sure it made more
> > sense to me at the time. I think I was trying to say that not only do
> > we not have a way to mask the physical hardware, but if we did, we don't
> > have a way to retrieve the pending bits, so any pending interrupts while
> > masked would be lost. We might be able to deal with that by posting a
> > spurious interrupt on unmask, but for now we do nothing as masking is
> > usually done just to update the vector. Thanks,
>
> Ok, thanks for the clarification.
>
> As we are at it, do you also recall if this
>
> --- a/hw/device-assignment.c
> +++ b/hw/device-assignment.c
> @@ -1573,28 +1573,7 @@ static void msix_mmio_write(void *opaque, target_phys_addr_t addr,
> */
> } else if (msix_masked(&orig) && !msix_masked(entry)) {
> /* Vector unmasked */
> - if (i >= adev->irq_entries_nr || !adev->entry[i].type) {
> - /* Previously unassigned vector, start from scratch */
> - assigned_dev_update_msix(pdev);
> - return;
> - } else {
> - /* Update an existing, previously masked vector */
> - struct kvm_irq_routing_entry orig = adev->entry[i];
> - int ret;
> -
> - adev->entry[i].u.msi.address_lo = entry->addr_lo;
> - adev->entry[i].u.msi.address_hi = entry->addr_hi;
> - adev->entry[i].u.msi.data = entry->data;
> -
> - ret = kvm_update_routing_entry(&orig, &adev->entry[i]);
> - if (ret) {
> - fprintf(stderr,
> - "Error updating irq routing entry (%d)\n", ret);
> - return;
> - }
> -
> - kvm_irqchip_commit_routes(kvm_state);
> - }
> + assigned_dev_update_msix(pdev);
> }
> }
> }
>
> would make a relevant difference for known workloads? I'm trying to get
> rid of direct routing table manipulations, but I would also like to
> avoid introducing things like kvm_irqchip_update_msi_route unless really
> necessary. Or could VFIO make use of that as well?
It makes me a little nervous, but I don't know that it won't work.
There's a lot more latency in turning off MSI-X and completely
rebuilding it than there is in updating the routing of a single vector.
You can imagine that irqbalance could be triggering this path pretty
regularly. Increasing vectors beyond what was previously setup is more
of an init-time event, so the latency doesn't bother me as much. We'd
probably have to send some spurious interrupts for anything we might
have missed if we take the high latency path.
VFIO is already a little more abstracted, making use of the msix vector
use and release interface, but we do still make use of the kvm_irqchip
irqfd/virq interfaces.
> PS: Will try to have a look at your main VFIO patch later today.
Thanks :)
Alex
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