Re: [Xen-devel] [PATCH v4 3/3] VT-d: Fix vt-d Device-TLB flush timeout issue.

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

> On January 27, 2016 at 9:15pm, <JBeulich@xxxxxxxx> wrote:
> >>> On 27.01.16 at 13:38, <quan.xu@xxxxxxxxx> wrote:
> >>  On January 27, 2016 at 7:24pm, <JBeulich@xxxxxxxx> wrote:
> >> >>> On 27.01.16 at 12:09, <quan.xu@xxxxxxxxx> wrote:
> >> >>  On January 27, 2016 at 6:48am, <Tian, Kevin> wrote:
> >> >> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> > Sent: Tuesday, January 26, 2016 11:53 PM
> >> >
> >> >
> >> >> > Once again: Before getting started, please assess which route is
> >> >> > going to be the better one. Remember that we had already
> >> >> > discussed and put aside some form of deferring the hiding of
> >> >> > devices, so if you come back with a patch doing that again,
> >> >> > you'll have to be able to explain why the alternative(s) are worse.
> >> >> >
> >> >>
> >> >> Quan, could you list pros/cons of those alternatives based on
> >> >> discussion so
> >> far?
> >> >> Then we can decide which way should be done before you go to
> >> >> actual
> >> coding.
> >> >> Earlier suggestion on hiding device immediately is under the
> >> >> assumption that all locks have been held. If this part becomes too
> >> >> complex, and you can explain clearly that deferring the hiding
> >> >> action doesn't lead to any race condition, then people can see why
> >> >> you are
> >> proposing defer again.
> >> >
> >> >
> >> > The following are pros/cons of those alternatives. It is also why I
> >> > propose defer again.
> >> >
> >> > -- --
> >> > 1. Hiding the devices immediately
> >> > Pros:
> >> >      * it makes whatever changes are ASAP after the Device-TLB flush
> error.
> >> >
> >> > Cons:
> >> >      * It may break the code path.
> >> >      * It may lead to any race condition.
> >> >      * Hiding the devices immediately is under the assumption that
> >> > all
> > locks
> >> have been held.
> >> >       Different locking state is possible for different call trees.
> >> > This
> > part
> >> becomes too complex.
> >>
> >> So you just repeat what you've already said before. "This part
> >> becomes too complex" you say without any kind of proof, yet that's
> >> what we need to understand whether the alternative of doing the
> >> locking correctly really is
> > this
> >> bad (and I continue to not see why it would).
> >
> >
> > Such as pcidevs_lock:
> >
> > 1. as I mentioned, it is indeed different locking state is possible
> > for different call trees of flush Device-TLB. When Device-TLB flush is
> > error, It is really challenge to judge whether to acquire the pcidevs_lock 
> > or
> not.
> >
> >    For example,
> >    *It is _not_under_ lock for the following call tree:
> > $ flush_iotlb_qi()--- iommu_flush_iotlb_psi() --
> > __intel_iommu_iotlb_flush()
> > --intel_iommu_iotlb_flush() --iommu_iotlb_flush()
> > --xenmem_add_to_physmap()--do_memory_op()
> >
> >    *It is _under_ lock for the following call tree:
> > $flush_iotlb_qi()--iommu_flush_iotlb_dsi()--domain_context_unmap_one()
> > --domain_con
> > text_unmap()--reassign_device_ownership()--deassign_device()-iommu_do_
> > pci_domctl()
> >
> > 2. if I try to acquire the pcidevs_lock for some _not_under_ lock call
> > tree, it makes things worse. As the pcidevs_lock is a big lock, then
> >   Frequent memory modification may block the pci-device assign due to
> > the pcidevs_lock. if I try to split the pcidevs_lock into small locks.
> >   It may takes a great deal of time to make it stable.
> 
> I don't understand this, namely in the context of my suggestion to simply pass
> down a flag indicating whether the lock is being held (and hence acquiring it
> only in the most narrow scope if not already owning it).
> 

This is also an idea.
BTW, Does the lock refer to pcidevs_lock?

-Quan










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