Re: [Qemu-devel] [RFC][PATCH 12/45] msi: Introduce MSIRoutingCache

[Jan Kiszka]

On 2011-10-17 17:37, Michael S. Tsirkin wrote:
> On Mon, Oct 17, 2011 at 01:19:56PM +0200, Jan Kiszka wrote:
>> On 2011-10-17 13:06, Avi Kivity wrote:
>>> On 10/17/2011 11:27 AM, Jan Kiszka wrote:
>>>> This cache will help us implementing KVM in-kernel irqchip support
>>>> without spreading hooks all over the place.
>>>>
>>>> KVM requires us to register it first and then deliver it by raising a
>>>> pseudo IRQ line returned on registration. While this could be changed
>>>> for QEMU-originated MSI messages by adding direct MSI injection, we will
>>>> still need this translation for irqfd-originated messages. The
>>>> MSIRoutingCache will allow to track those registrations and update them
>>>> lazily before the actual delivery. This avoid having to track MSI
>>>> vectors at device level (like qemu-kvm currently does).
>>>>
>>>>
>>>> +typedef enum {
>>>> +    MSI_ROUTE_NONE = 0,
>>>> +    MSI_ROUTE_STATIC,
>>>> +} MSIRouteType;
>>>> +
>>>> +struct MSIRoutingCache {
>>>> +    MSIMessage msg;
>>>> +    MSIRouteType type;
>>>> +    int kvm_gsi;
>>>> +    int kvm_irqfd;
>>>> +};
>>>> +
>>>> diff --git a/hw/pci.h b/hw/pci.h
>>>> index 329ab32..5b5d2fd 100644
>>>> --- a/hw/pci.h
>>>> +++ b/hw/pci.h
>>>> @@ -197,6 +197,10 @@ struct PCIDevice {
>>>>      MemoryRegion rom;
>>>>      uint32_t rom_bar;
>>>>  
>>>> +    /* MSI routing chaches */
>>>> +    MSIRoutingCache *msi_cache;
>>>> +    MSIRoutingCache *msix_cache;
>>>> +
>>>>      /* MSI entries */
>>>>      int msi_entries_nr;
>>>>      struct KVMMsiMessage *msi_irq_entries;
>>>
>>> IMO this needlessly leaks kvm information into core qemu.  The cache
>>> should be completely hidden in kvm code.
>>>
>>> I think msi_deliver() can hide the use of the cache completely.  For
>>> pre-registered events like kvm's irqfd, you can use something like
>>>
>>>   qemu_irq qemu_msi_irq(MSIMessage msg)
>>>
>>> for non-kvm, it simply returns a qemu_irq that triggers a stl_phys();
>>> for kvm, it allocates an irqfd and a permanent entry in the cache and
>>> returns a qemu_irq that triggers the irqfd.
>>
>> See my previously mail: you want to track the life-cycle of an MSI
>> source to avoid generating routes for identical sources. A messages is
>> not a source. Two identical messages can come from different sources.
> 
> Since MSI messages are edge triggered, I don't see how this
> would work without losing interrupts. And AFAIK,
> existing guests do not use the same message for
> different sources.

Just like we handle shared edge-triggered line-base IRQs, shared MSIs
are in principle feasible as well.

Jan

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