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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.01.16 at 15:13, <quan.xu@xxxxxxxxx> wrote:
>>  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?

Yes, for now I assume that only that lock actually needs special
attention.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel