On Wed, 2015-10-28 at 10:50 -0700, Yunhong Jiang wrote:
> On Wed, Oct 28, 2015 at 01:44:55AM +0100, Paolo Bonzini wrote:
> >
> >
> > On 27/10/2015 22:26, Yunhong Jiang wrote:
> > >> > On RT kernels however can you call eventfd_signal from interrupt
> > >> > context? You cannot call spin_lock_irqsave (which can sleep) from a
> > >> > non-threaded interrupt handler, can you? You would need a raw spin
> > >> > lock.
> > > Thanks for pointing this out. Yes, we can't call spin_lock_irqsave on RT
> > > kernel. Will do this way on next patch. But not sure if it's overkill to
> > > use
> > > raw_spinlock there since the eventfd_signal is used by other caller also.
> >
> > No, I don't think you can use raw_spinlock there. The problem is not
> > just eventfd_signal, it is especially wake_up_locked_poll. You cannot
> > convert the whole workqueue infrastructure to use raw_spinlock.
>
> You mean the waitqueue, instead of workqueue, right? One choice is to change
> the eventfd to use simple wait queue, which is raw_spinlock. But use simple
> waitqueue on eventfd may in fact impact real time latency if not in this
> scenario.
>
> >
> > Alex, would it make sense to use the IRQ bypass infrastructure always,
> > not just for VT-d, to do the MSI injection directly from the VFIO
> > interrupt handler and bypass the eventfd? Basically this would add an
> > RCU-protected list of consumers matching the token to struct
> > irq_bypass_producer, and a
> >
> > int (*inject)(struct irq_bypass_consumer *);
> >
> > callback to struct irq_bypass_consumer. If any callback returns true,
> > the eventfd is not signaled. The KVM implementation would be like this
> > (compare with virt/kvm/eventfd.c):
> >
> > /* Extracted out of irqfd_wakeup */
> > static int
> > irqfd_wakeup_pollin(struct kvm_kernel_irqfd *irqfd)
> > {
> > ...
> > }
> >
> > /* Extracted out of irqfd_wakeup */
> > static int
> > irqfd_wakeup_pollhup(struct kvm_kernel_irqfd *irqfd)
> > {
> > ...
> > }
> >
> > static int
> > irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync,
> > void *key)
> > {
> > struct _irqfd *irqfd = container_of(wait,
> > struct _irqfd, wait);
> > unsigned long flags = (unsigned long)key;
> >
> > if (flags & POLLIN)
> > irqfd_wakeup_pollin(irqfd);
> > if (flags & POLLHUP)
> > irqfd_wakeup_pollhup(irqfd);
> >
> > return 0;
> > }
> >
> > static int kvm_arch_irq_bypass_inject(
> > struct irq_bypass_consumer *cons)
> > {
> > struct kvm_kernel_irqfd *irqfd =
> > container_of(cons, struct kvm_kernel_irqfd,
> > consumer);
> >
> > irqfd_wakeup_pollin(irqfd);
> > }
> >
> This is a good idea IMHO. So for MSI interrupt, the
> kvm_arch_irq_bypass_inject will be used, and the irqfd_wakeup will not be
> invoked anymore, am I right?
>
> I noticed the irq bypass manager is not merged yet, are there any git branch
> for it?
It's in linux-next via the kvm.git next branch:
git://git.kernel.org/pub/scm/virt/kvm/kvm.git
Thanks,
Alex
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