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RE: [Xen-devel] [PATCH] Re-enable MSI support

>From: Espen Skoglund
>Sent: Wednesday, December 10, 2008 7:34 PM
>[Yunhong Jiang]
>> This patch try to do some cleanup for these issues.
>> 1) The basic idea is to remove the pci_dev's lock, instead, we try
>>    to use the big pcidevs_lock to protect the whole pci_dev
>>    stuff. It including both pci_dev adding/removing, and also the
>>    assignment of the devices. We checked the code and seems there is
>>    no critical code path for this. We try to use fine-grained lock
>>    and seems the code will be very tricky.
>> 2) Split the pci_enable_msi into two step, firstly it will just
>>    construct the msi_desc through pci_enable_msi without holding the
>>    irq_desc lock, and then it will setup msi through setup_msi_irq
>>    with irq_desc holded.
>> 3) Change the iommu->lock and hd->mapping_lock to be irq_save. 
>> 4) Fix to some minor issues.
>> Now the lock sequence is: pcidevs_lock -> domai's event_lock ->
>> iommu's lock -> hvm_iommu's mapping_lock. The irq_desc's lock will
>> always be the last lock be hold for peformance consideration.
>So what exactly is it that pcidevs_lock is supposed to "protect" now?
>Does it indicate that someone is holding a reference to a pci_dev?
>Does it indicate that someone will modify some pci_dev?  Does it
>indicate that someone is messing around with interrupts or MSI

I think it protects all above. As those operations are all rare, such a
big lock can avoid complex lock/unlock sequence regarding to different
paths to different resource of an assigned device.

>Regarding pci_enable_msi: Can you not get into race conditions with
>the splitup you're doing?  pci_enable_msi() does afterall poke around
>in the MSI capability record.
>       eSk

If I read it correctly, only one out of lock is to mask/unmask MSI
vector. MSI-X has per-vector mask dword, and thus no contention.
multiple-vector support for MSI is not supported yet. All other touch
to MSI/MSI-X capabilities are protected by that lock.

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