Date:   Thu, 29 Aug 2019 07:48:42 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Ben Luo <luoben@...ux.alibaba.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        tao.ma@...ux.alibaba.com, gerry@...ux.alibaba.com,
        nanhai.zou@...ux.alibaba.com, linyunsheng@...wei.com
Subject: Re: [PATCH v4 3/3] vfio/pci: make use of irq_update_devid and
 optimize irq ops

On Thu, 29 Aug 2019 13:40:59 +0800
Ben Luo <luoben@...ux.alibaba.com> wrote:

> 在 2019/8/29 上午1:23, Alex Williamson 写道:
> > On Wed, 28 Aug 2019 18:08:02 +0800
> > Ben Luo <luoben@...ux.alibaba.com> wrote:
> >  
> >> 在 2019/8/28 上午4:33, Alex Williamson 写道:  
> >>> On Thu, 22 Aug 2019 23:34:43 +0800
> >>> Ben Luo <luoben@...ux.alibaba.com> wrote:
> >>>     
> >>>> When userspace (e.g. qemu) triggers a switch between KVM
> >>>> irqfd and userspace eventfd, only dev_id of irq action
> >>>> (i.e. the "trigger" in this patch's context) will be
> >>>> changed, but a free-then-request-irq action is taken in
> >>>> current code. And, irq affinity setting in VM will also
> >>>> trigger a free-then-request-irq action, which actually
> >>>> changes nothing, but only fires a producer re-registration
> >>>> to update irte in case that posted-interrupt is in use.
> >>>>
> >>>> This patch makes use of irq_update_devid() and optimize
> >>>> both cases above, which reduces the risk of losing interrupt
> >>>> and also cuts some overhead.
> >>>>
> >>>> Signed-off-by: Ben Luo <luoben@...ux.alibaba.com>
> >>>> ---
> >>>>    drivers/vfio/pci/vfio_pci_intrs.c | 112 +++++++++++++++++++++++++-------------
> >>>>    1 file changed, 74 insertions(+), 38 deletions(-)
> >>>>
> >>>> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> index 3fa3f72..60d3023 100644
> >>>> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> @@ -284,70 +284,106 @@ static int vfio_msi_enable(struct vfio_pci_device *vdev, int nvec, bool msix)
> >>>>    static int vfio_msi_set_vector_signal(struct vfio_pci_device *vdev,
> >>>>    				      int vector, int fd, bool msix)
> >>>>    {
> >>>> +	struct eventfd_ctx *trigger = NULL;
> >>>>    	struct pci_dev *pdev = vdev->pdev;
> >>>> -	struct eventfd_ctx *trigger;
> >>>>    	int irq, ret;
> >>>>    
> >>>>    	if (vector < 0 || vector >= vdev->num_ctx)
> >>>>    		return -EINVAL;
> >>>>    
> >>>> +	if (fd >= 0) {
> >>>> +		trigger = eventfd_ctx_fdget(fd);
> >>>> +		if (IS_ERR(trigger))
> >>>> +			return PTR_ERR(trigger);
> >>>> +	}  
> >>> I think this is a user visible change.  Previously the vector is
> >>> disabled first, then if an error occurs re-enabling, we return an errno
> >>> with the vector disabled.  Here we instead fail the ioctl and leave the
> >>> state as if it had never happened.  For instance with QEMU, if they
> >>> were trying to change from KVM to userspace signaling and entered this
> >>> condition, previously the interrupt would signal to neither eventfd, now
> >>> it would continue signaling to KVM. If QEMU's intent was to emulate
> >>> vector masking, this could induce unhandled interrupts in the guest.
> >>> Maybe we need a tear-down on fault here to maintain that behavior, or
> >>> do you see some justification for the change?  
> >> Thanks for your comments, this reminds me to think more about the
> >> effects to users.
> >>
> >> After I reviewed the related code in Qemu and VFIO, I think maybe there
> >> is a problem in current behavior
> >> for the signal path changing case. Qemu has neither recovery nor retry
> >> code in case that ioctl with
> >> 'VFIO_DEVICE_SET_IRQS' command fails, so if the old signal path has been
> >> disabled on fault of setting
> >> up new path, the corresponding vector may be disabled forever. Following
> >> is an example from qemu's
> >> vfio_msix_vector_do_use():
> >>
> >>           ret = ioctl(vdev->vbasedev.fd, VFIO_DEVICE_SET_IRQS, irq_set);
> >>           g_free(irq_set);
> >>           if (ret) {
> >>               error_report("vfio: failed to modify vector, %d", ret);
> >>           }
> >>
> >> I think the singal path before changing should be still working at this
> >> moment and the caller should keep it
> >> working if the changing fails, so that at least we still have the old
> >> path instead of no path.
> >>
> >> For masking vector case, the 'fd' should be -1, and the interrupt will
> >> be freed as before this patch.  
> > QEMU doesn't really have an opportunity to signal an error to the
> > guest, we're emulating the hardware masking of MSI and MSI-X.  The
> > guest is simply trying to write a mask bit in the vector, there's no
> > provision in the PCI spec that setting this bit can fail.  The current
> > behavior is that the vector is disabled on error.  We can argue whether
> > that's the optimal behavior, but it's the existing behavior and
> > changing it would require and evaluation of all existing users.  
> 
> I totally agree with you that masking of MSI and MSI-X should follow 
> current behavior, and my code does follow this behavior when 'fd' == -1, 
> the interrupt will surely be disabled.
> 
> There is another case that 'fd' is not -1, means userspace just want to 
> change the singal path, this time we do have a chance of encountering 
> error on eventfd_ctx_fdget(fd). So, I think it is better to keep the old 
> path working in this case.
> 
> But, as you said this may break the expectation of existing users, I 
> make it tear-down on fault in next version.
> 
> >  
> >>>> +
> >>>>    	irq = pci_irq_vector(pdev, vector);
> >>>>    
> >>>> +	/*
> >>>> +	 * For KVM-VFIO case, interrupt from passthrough device will be directly
> >>>> +	 * delivered to VM after producer and consumer connected successfully.
> >>>> +	 * If producer and consumer are disconnected, this interrupt process
> >>>> +	 * will fall back to remap mode, where interrupt handler uses 'trigger'
> >>>> +	 * to find the right way to deliver the interrupt to VM. So, it is safe
> >>>> +	 * to do irq_update_devid() before irq_bypass_unregister_producer() which
> >>>> +	 * switches interrupt process to remap mode. To producer and consumer,
> >>>> +	 * 'trigger' is only a token used for pairing them togather.
> >>>> +	 */
> >>>>    	if (vdev->ctx[vector].trigger) {
> >>>> -		free_irq(irq, vdev->ctx[vector].trigger);
> >>>> -		irq_bypass_unregister_producer(&vdev->ctx[vector].producer);
> >>>> -		kfree(vdev->ctx[vector].name);
> >>>> -		eventfd_ctx_put(vdev->ctx[vector].trigger);
> >>>> -		vdev->ctx[vector].trigger = NULL;
> >>>> +		if (vdev->ctx[vector].trigger == trigger) {
> >>>> +			/* switch back to remap mode */
> >>>> +			irq_bypass_unregister_producer(&vdev->ctx[vector].producer);  
> >>> I think we leak the fd context we acquired above in this case.  
> >> Thanks for pointing it out, I will fix this in next version.  
> >>> Why do we do anything in this case, couldn't we just 'put' the extra ctx
> >>> and return 0 here?  
> >> Sorry for confusing and I do this for a reason,  I will add some more
> >> comments like this:
> >>
> >> Unregistration here is for re-resigtraion later, which will trigger the
> >> reconnection of irq_bypass producer
> >> and consumer, which in turn calls vmx_update_pi_irte() to update IRTE if
> >> posted interrupt is in use.
> >> (vmx_update_pi_irte() will modify IRTE based on the information
> >> retrieved from KVM.)
> >> Whether producer token changed or not, irq_bypass_register_producer() is
> >> a way (seems the only way) to
> >> update IRTE by VFIO for posted interrupt. The IRTE will be used by IOMMU
> >> directly to find the target CPU
> >> for an interrupt posted to VM, since hypervisor is bypassed.  
> > This is only explaining what the bypass de-registration and
> > re-registration does, not why we need to perform those actions here.
> > If the trigger is the same as that already attached to this vector, why
> > is the IRTE changing?  Seems this ought to be a no-op for this vector.  
> 
> Sorry, I think it cannot be a no-op here. The interrupt affinity setting 
> in guest also triggers the calling of this function and IRTE will be 
> modified with new affinity information retrieved from KVM's data 
> structure by vmx_update_pi_irte() when posted interrupt is in use, not 
> from the 'trigger'.

Aha, this is the key piece of information I was missing.  Please add a
comment that even if the trigger is unchanged we need to bounce the
bypass to allow affinity changes in the guest to be realized.  Thanks,

Alex

> >>>> +		} else if (trigger) {
> >>>> +			ret = irq_update_devid(irq,
> >>>> +					       vdev->ctx[vector].trigger, trigger);
> >>>> +			if (unlikely(ret)) {
> >>>> +				dev_info(&pdev->dev,
> >>>> +					 "update devid of %d (token %p) failed: %d\n",
> >>>> +					 irq, vdev->ctx[vector].trigger, ret);
> >>>> +				eventfd_ctx_put(trigger);
> >>>> +				return ret;
> >>>> +			}
> >>>> +			irq_bypass_unregister_producer(&vdev->ctx[vector].producer);  
> >>> Can you explain this ordering, I would have expected that we'd
> >>> unregister the bypass before we updated the devid.  Thanks,
> >>>
> >>> Alex  
> >> Actually, I have explained this in comments above this whole control
> >> block. I think it is safe and better to
> >> update devid before irq_bypass_unregister_producer() which switches
> >> interrupt process from posted mode
> >> to remap mode. So, if update fails, the posted interrupt can still work.
> >>
> >> Anyway, to producer and consumer,  'trigger' is only a token used for
> >> pairing them togather.  
> > The bypass is not a guaranteed mechanism, it's an opportunistic
> > accelerator.  If the devid update fails, what state are we left with?
> > The irq action may not work, but the bypass does; maybe; maybe not all
> > the time?  This seems to fall into the same consistency in userspace
> > behavior issue identified above.  The user ABI is that the vector is
> > disabled if an error is returned.  Thanks,
> >
> > Alex  
> 
> Thanks, now I see it is better to unregister the bypass before update 
> the devid, will change ordering in next version.
> 
> Ben
> 

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