[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 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
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