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SubjectRe: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts
On 2012-06-04 13:21, Thomas Gleixner wrote:
> On Sun, 3 Jun 2012, Avi Kivity wrote:
>
>> On 06/01/2012 09:26 PM, Jan Kiszka wrote:
>>>
>>>> you suggesting we need a request_edge_threaded_only_irq() API? Thanks,
>>>
>>> I'm just wondering if that restriction for threaded IRQs is really
>>> necessary for all use cases we have. Threaded MSIs do not appear to me
>>> like have to be handled that conservatively, but maybe I'm missing some
>>> detail.
>>>
>>
>> btw, I'm hoping we can unthread assigned MSIs. If the delivery is
>> unicast, we can precalculate everything and all the handler has to do is
>> set the IRR, KVM_REQ_EVENT, and kick the vcpu. All of these can be done
>> from interrupt context with just RCU locking.
>
> There is really no need to run MSI/MSI-X interrupts threaded for
> KVM. I'm running the patch below for quite some time and it works like
> a charm.
>
> Thanks,
>
> tglx
> ----
> Index: linux-2.6/virt/kvm/assigned-dev.c
> ===================================================================
> --- linux-2.6.orig/virt/kvm/assigned-dev.c
> +++ linux-2.6/virt/kvm/assigned-dev.c
> @@ -105,7 +105,7 @@ static irqreturn_t kvm_assigned_dev_thre
> }
>
> #ifdef __KVM_HAVE_MSI
> -static irqreturn_t kvm_assigned_dev_thread_msi(int irq, void *dev_id)
> +static irqreturn_t kvm_assigned_dev_msi_handler(int irq, void *dev_id)
> {
> struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
>
> @@ -117,7 +117,7 @@ static irqreturn_t kvm_assigned_dev_thre
> #endif
>
> #ifdef __KVM_HAVE_MSIX
> -static irqreturn_t kvm_assigned_dev_thread_msix(int irq, void *dev_id)
> +static irqreturn_t kvm_assigned_dev_msix_handler(int irq, void *dev_id)
> {
> struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
> int index = find_index_from_host_irq(assigned_dev, irq);
> @@ -346,9 +346,8 @@ static int assigned_device_enable_host_m
> }
>
> dev->host_irq = dev->dev->irq;
> - if (request_threaded_irq(dev->host_irq, NULL,
> - kvm_assigned_dev_thread_msi, 0,
> - dev->irq_name, dev)) {
> + if (request_irq(dev->host_irq, kvm_assigned_dev_msi_handler, 0,
> + dev->irq_name, dev)) {
> pci_disable_msi(dev->dev);
> return -EIO;
> }
> @@ -373,9 +372,9 @@ static int assigned_device_enable_host_m
> return r;
>
> for (i = 0; i < dev->entries_nr; i++) {
> - r = request_threaded_irq(dev->host_msix_entries[i].vector,
> - NULL, kvm_assigned_dev_thread_msix,
> - 0, dev->irq_name, dev);
> + r = request_irq(dev->host_msix_entries[i].vector,
> + kvm_assigned_dev_msix_handler, 0,
> + dev->irq_name, dev);
> if (r)
> goto err;
> }

This may work in practice but has two conceptual problems:
- we do not want to run a potential broadcast to all VCPUs to run in
a host IRQ handler
- crazy user space could have configured the route to end up in the
PIC or IOAPIC, and both are not hard-IRQ safe (this should probably
be caught on setup)

So this shortcut requires some checks before being applied to a specific
MSI/MSI-X vector.


Taking KVM aside, my general question remains if threaded MSI handlers
of all devices really need to apply IRQF_ONESHOT though they should have
no use for it.

Jan

--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux


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