Messages in this thread | | | Subject | Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces | From | Lu Baolu <> | Date | Sat, 28 Sep 2019 16:23:16 +0800 |
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Hi Peter,
On 9/27/19 1:34 PM, Peter Xu wrote: > Hi, Baolu, > > On Fri, Sep 27, 2019 at 10:27:24AM +0800, Lu Baolu wrote: >>>>>> + spin_lock(&(domain)->page_table_lock); \ >>>>> >>>>> Is this intended to lock here instead of taking the lock during the >>>>> whole page table walk? Is it safe? >>>>> >>>>> Taking the example where nm==PTE: when we reach here how do we >>>>> guarantee that the PMD page that has this PTE is still valid? >>>> >>>> We will always keep the non-leaf pages in the table, >>> >>> I see. Though, could I ask why? It seems to me that the existing 2nd >>> level page table does not keep these when unmap, and it's not even use >>> locking at all by leveraging cmpxchg()? >> >> I still need some time to understand how cmpxchg() solves the race issue >> when reclaims pages. For example. >> >> Thread A Thread B >> -A1: check all PTE's empty -B1: up-level PDE valid >> -A2: clear the up-level PDE >> -A3: reclaim the page -B2: populate the PTEs >> >> Both (A1,A2) and (B1,B2) should be atomic. Otherwise, race could happen. > > I'm not sure of this, but IMHO it is similarly because we need to > allocate the iova ranges from iova allocator first, so thread A (who's > going to unmap pages) and thread B (who's going to map new pages) > should never have collapsed regions if happening concurrently. I'm
Although they don't collapse, they might share a same pmd entry. If A cleared the pmd entry and B goes ahead with populating the pte's. It will crash.
> referring to intel_unmap() in which we won't free the iova region > before domain_unmap() completes (which should cover the whole process > of A1-A3) so the same iova range to be unmapped won't be allocated to > any new pages in some other thread. > > There's also a hint in domain_unmap(): > > /* we don't need lock here; nobody else touches the iova range */ > >> >> Actually, the iova allocator always packs IOVA ranges close to the top >> of the address space. This results in requiring a minimal number of >> pages to map the allocated IOVA ranges, which makes memory onsumption >> by IOMMU page tables tolerable. Hence, we don't need to reclaim the >> pages until the whole page table is about to tear down. The real data >> on my test machine also improves this. > > Do you mean you have run the code with a 1st-level-supported IOMMU > hardware? IMHO any data point would be good to be in the cover letter > as reference.
Yes. Sure! Let me do this since the next version.
> > [...] > >>>>>> +static struct page * >>>>>> +mmunmap_pte_range(struct dmar_domain *domain, pmd_t *pmd, >>>>>> + unsigned long addr, unsigned long end, >>>>>> + struct page *freelist, bool reclaim) >>>>>> +{ >>>>>> + int i; >>>>>> + unsigned long start; >>>>>> + pte_t *pte, *first_pte; >>>>>> + >>>>>> + start = addr; >>>>>> + pte = pte_offset_kernel(pmd, addr); >>>>>> + first_pte = pte; >>>>>> + do { >>>>>> + set_pte(pte, __pte(0)); >>>>>> + } while (pte++, addr += PAGE_SIZE, addr != end); >>>>>> + >>>>>> + domain_flush_cache(domain, first_pte, (void *)pte - (void *)first_pte); >>>>>> + >>>>>> + /* Add page to free list if all entries are empty. */ >>>>>> + if (reclaim) { >>>>> >>>>> Shouldn't we know whether to reclaim if with (addr, end) specified as >>>>> long as they cover the whole range of this PMD? >>>> >>>> Current policy is that we don't reclaim any pages until the whole page >>>> table will be torn down. >>> >>> Ah OK. But I saw that you're passing in relaim==!start_addr. >>> Shouldn't that errornously trigger if one wants to unmap the 1st page >>> as well even if not the whole address space? >> >> IOVA 0 is assumed to be reserved by the allocator. Otherwise, we have no >> means to check whether a IOVA is valid. > > Is this an assumption of the allocator? Could that change in the future?
Yes. And I think it should keep unless no consumer depends on this optimization.
> > IMHO that's not necessary if so, after all it's as simple as replacing > (!start_addr) with (start == 0 && end == END). I see that in > domain_unmap() it has a similar check when freeing pgd: > > if (start_pfn == 0 && last_pfn == DOMAIN_MAX_PFN(domain->gaw)) >
Yours looks better. Thank you!
> Thanks, >
Best regards, Baolu
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