Re: [PATCH 0/5] vfio/pci: Fix up breakage against split irqchip and INTx

[Peter Xu]

On Thu, Feb 27, 2020 at 04:32:24PM +0100, Auger Eric wrote:
> Hi Peter,

Hi, Eric,

> 
> On 2/26/20 11:50 PM, Peter Xu wrote:
> > VFIO INTx is not working with split irqchip.  On new kernels KVM_IRQFD
> > will directly fail with resamplefd attached so QEMU will automatically
> > fallback to the INTx slow path.  However on old kernels it's still
> > broken.
> I think you should quote the commit that changes the behavior:
> 654f1f13ea56  kvm: Check irqchip mode before assign irqfd?
> so that one can understand what it fixed.

Right I should mention that.

Actually I mentioned it in an old cover letter but unluckily something
wrong happened with git-publish and merely my whole cover letter is
gone...  So what you saw is actually a simplified version and I think
I must have lost something like this... :(

Thanks for bringing that up.

> 
> So on kernels that do not feature that commit, the ioctl succeeds and we
> pretend the resamplefd was properly attached whereas in practice it was
> never triggered.

Right.  Another thing I just noticed is that we must _not_ attach the
resamplefd to the KVM_IRQFD ioctl when we registered this in the
userspace.  One thing is it's for sure not needed at all.  More
importantly, on new kernels (after commit 654f1f13ea56) the KVM_IRQFD
could fail (because it sees both split irqchip and resamplefd) and
QEMU will fallback again to the slow path even if the fast path can
work.

Thanks,

-- 
Peter Xu