Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH 5/7] x86/mmu: Allocate/free PASID | Date | Tue, 28 Apr 2020 20:54:01 +0200 |
| |
"Jacob Pan (Jun)" <jacob.jun.pan@intel.com> writes: > On Sun, 26 Apr 2020 16:55:25 +0200 > Thomas Gleixner <tglx@linutronix.de> wrote: >> Fenghua Yu <fenghua.yu@intel.com> writes: >> > The PASID is freed when the process exits (so no need to keep >> > reference counts on how many SVM devices are sharing the PASID). >> >> I'm not buying that. If there is an outstanding request with the PASID >> of a process then tearing down the process address space and freeing >> the PASID (which might be reused) is fundamentally broken. >> > Device driver unbind PASID is tied to FD release. So when a process > exits, FD close causes driver to do the following: > > 1. stops DMA > 2. unbind PASID (clears the PASID entry in IOMMU, flush all TLBs, drain > in flight page requests)
Fair enough. Explaining that somewhere might be helpful.
> For bare metal SVM, if the last mmdrop always happens after FD release, > we can ensure no outstanding requests at the point of ioasid_free(). > Perhaps this is a wrong assumption?
If fd release cleans up then how should there be something in flight at the final mmdrop?
> For guest SVM, there will be more users of a PASID. I am also > working on adding refcounting to ioasid. ioasid_free() will not release > the PASID back to the pool until all references are dropped.
What does more users mean?
>> > + if (mm && mm->context.pasid && !(flags & >> > SVM_FLAG_PRIVATE_PASID)) { >> > + /* >> > + * Once a PASID is allocated for this mm, the PASID >> > + * stays with the mm until the mm is dropped. Reuse >> > + * the PASID which has been already allocated for >> > the >> > + * mm instead of allocating a new one. >> > + */ >> > + ioasid_set_data(mm->context.pasid, svm); >> >> So if the PASID is reused several times for different SVMs then every >> time ioasid_data->private is set to a different SVM. How is that >> supposed to work? >> > For the lifetime of the mm, there is only one PASID. svm_bind/unbind_mm > could happen many times with different private data: intel_svm. > Multiple devices can bind to the same PASID as well. But private data > don't change within the first bind and last unbind.
Ok. I read through that spaghetti of intel_svm_bind_mm() again and now I start to get an idea how that is supposed to work. What a mess.
That function really wants to be restructured in a way so it is understandable to mere mortals.
Thanks,
tglx
| |