[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v4 3/3] VT-d: Fix vt-d Device-TLB flush timeout issue.
> 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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