Re: [PATCH] KVM: Allow host IRQ sharing for assigned PCI 2.3 devices

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-01-10 19:31, Michael S. Tsirkin wrote:
> On Tue, Jan 10, 2012 at 07:21:01PM +0100, Jan Kiszka wrote:
>>> ATM writes to msi/msix mask bit have no effect for assigned
>>> devices. For virtio, they are implemented by deassigning irqfd
>>> which is a very slow operation (rcu write side).
>>>
>>> Instead, When guest writes to mask, qemu can set/clear by calling
>>> this ioctl.
>>
>> Isn't that effort better invested in proper in-kernel mask emulation for
>> MSI-X?
> 
> This gives us a working implementation fo free.  Whether MSIX mask
> writes are worth accelerating in kernel I'm not 100% sure.

If it's worth optimizing the irqfd on/off dance, then it's more than
likely that eliminating the heavy user space exits, additional syscalls
along that way, and locking contentions up there is worth it as well. We
even have those mask ops in a time-critical paths here, unfortunately.

>  But IMO this
> shows it is a more generic interface.

I'm worried about adding something new that will soon become obsolete
again. That's wasted effort IMHO unless we say today that there will be
no in-kernel MSI-X support.

Jan

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