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.
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);
}
Or do you think it would be a hack? The latency improvement might
actually be even better than what Yunhong is already reporting.
Paolo
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