On Tue, Jan 10, 2012 at 07:43:36PM +0100, Jan Kiszka wrote: > 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, Not sure about that either. At least for virtio in my tests they almost never trigger. But it's needed for correctness for assigned devices for msix. > 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 Yes. But as we are adding a new interface maybe it's better to add a more generic one? I don't insist as I don't have a specific proposal, just something to consider. > -- > Siemens AG, Corporate Technology, CT T DE IT 1 > Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
