Re: [Xen-devel] [Patch v11 3/3] vt-d: fix vt-d Device-TLB flush timeout issue

["Xu, Quan" <quan.xu@xxxxxxxxx>]

On June 16, 2016 5:05 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>> On 16.06.16 at 10:42, <quan.xu@xxxxxxxxx> wrote:
> > On June 02, 2016 7:07 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>> On 01.06.16 at 11:05, <quan.xu@xxxxxxxxx> wrote:
> >> > +static void dev_invalidate_iotlb_timeout(struct iommu *iommu, u16 did,
> >> > +                                         struct pci_ats_dev
> >> > +*ats_dev) {
> >> > +    struct domain *d = NULL;
> >> > +    struct pci_dev *pdev;
> >> > +
> >> > +    if ( test_bit(did, iommu->domid_bitmap) )
> >> > +        d = rcu_lock_domain_by_id(iommu->domid_map[did]);
> >> > +
> >> > +    /*
> >> > +     * In case the domain has been freed or the IOMMU domid bitmap is
> >> > +     * not valid, the device no longer belongs to this domain.
> >> > +     */
> >> > +    if ( d == NULL )
> >> > +        return;
> >> > +
> >> > +    pcidevs_lock();
> >> > +
> >> > +    for_each_pdev(d, pdev)
> >> > +    {
> >> > +        if ( (pdev->seg == ats_dev->seg) &&
> >> > +             (pdev->bus == ats_dev->bus) &&
> >> > +             (pdev->devfn == ats_dev->devfn) )
> >> > +        {
> >> > +            ASSERT(pdev->domain);
> >> > +            list_del(&pdev->domain_list);
> >> > +            pdev->domain = NULL;
> >> > +            pci_hide_existing_device(pdev);
> >> > +            break;
> >> > +        }
> >> > +    }
> >> > +
> >> > +    pcidevs_unlock();
> >>
> >> ... this loop (and locking). (Of course such a change may better be
> >> done in another preparatory patch.)
> >>
> >
> > To eliminate the locking?  I am afraid the locking is still a must
> > here even without the loop, also referring  to device_assigned()..
> 
> If the entire loop gets eliminated, what would be left is
> 
>     pcidevs_lock();
>     pcidevs_unlock();
> 
> which I don't think does any good.
> 

Why? I can't follow it..


> >> > +static int __must_check dev_invalidate_sync(struct iommu *iommu,
> >> > +u16
> >> did,
> >> > +                                            struct pci_ats_dev
> >> > +*ats_dev) {
> >> > +    struct qi_ctrl *qi_ctrl = iommu_qi_ctrl(iommu);
> >> > +    int rc = 0;
> >> > +
> >> > +    if ( qi_ctrl->qinval_maddr )
> >> > +    {
> >> > +        rc = queue_invalidate_wait(iommu, 0, 1, 1, 1);
> >> > +
> >> > +        if ( rc == -ETIMEDOUT )
> >> > +            dev_invalidate_iotlb_timeout(iommu, did, ats_dev);
> >> > +    }
> >> > +
> >> > +    return rc;
> >> > +}
> >>
> >> I've never really understood why invalidate_sync() returns success
> >> when it didn't do anything. Now that you copy this same behavior
> >> here, I really need to ask you to explain that.
> >>
> >
> > It is acceptable to me, returning success when it didn't do anything
> > -- this is worth reflection and criticism:(..
> > It is better:
> > +    if ( qi_ctrl->qinval_maddr )
> > +        ...
> > +    else
> > +        rc = -ENOENT;
> 
> Right. And perhaps a separate patch to make invalidate_sync() do the same.

Agreed.

> Question is whether this really ought to be a conditional, or whether instead
> this code is unreachable when qinval is not in use, in which case these
> conditionals would imo better be converted to ASSERT()s.
> 

IMO, this should be a conditional.
As mentioned below, strictly speaking, this is a bug. We can't  ASSERT() based 
on a bug..
A coming patch may fix it..


> > A question:
> > I find the page related to qi_ctrl->qinval_maddr is not freed at all.
> > IMO, In disable_qinval (), we need to do:
> >      - free the page related to qi_ctrl->qinval_maddr.
> >      - qi_ctrl->qinval_maddr = 0;
> 
> Well, that's a correct observation, but not a big problem imo: If this was a 
> per-
> domain resource, it surely would need fixing. But if freeing a couple of these
> pages (one per IOMMU) causes synchronization headaches (e.g. because
> there may still be dangling references to it), then I think freeing them is 
> not a
> must. But if freeing them is safe (like you seem to imply), then I'm certainly
> fine with you fixing this (not that my opinion would matter much here, as I'm
> not the maintainer of this code).
> 

Agreed, thanks for your explanation. At least I will leave it as is in this 
patch set.

Quan

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