On 2012-08-14 16:31, Alex Williamson wrote:
> 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.
Yeah, good points.
>
> 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.
Hmm, but due to the nature of the callbacks, we always disable/reanable
on mask/unmask. So VFIO will be slower than current device assignment in
this regard.
BTW, how do you handle the device's PBA? Pass it through to the guest?
Jan
--
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux
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