Messages in this thread | | | Subject | Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature | From | David Hildenbrand <> | Date | Wed, 31 Mar 2021 11:32:46 +0200 |
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On 31.03.21 11:21, Catalin Marinas wrote: > On Wed, Mar 31, 2021 at 09:34:44AM +0200, David Hildenbrand wrote: >> On 30.03.21 12:30, Catalin Marinas wrote: >>> On Mon, Mar 29, 2021 at 05:06:51PM +0100, Steven Price wrote: >>>> On 28/03/2021 13:21, Catalin Marinas wrote: >>>>> On Sat, Mar 27, 2021 at 03:23:24PM +0000, Catalin Marinas wrote: >>>>>> On Fri, Mar 12, 2021 at 03:18:58PM +0000, Steven Price wrote: >>>>>>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c >>>>>>> index 77cb2d28f2a4..b31b7a821f90 100644 >>>>>>> --- a/arch/arm64/kvm/mmu.c >>>>>>> +++ b/arch/arm64/kvm/mmu.c >>>>>>> @@ -879,6 +879,22 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, >>>>>>> if (vma_pagesize == PAGE_SIZE && !force_pte) >>>>>>> vma_pagesize = transparent_hugepage_adjust(memslot, hva, >>>>>>> &pfn, &fault_ipa); >>>>>>> + >>>>>>> + if (fault_status != FSC_PERM && kvm_has_mte(kvm) && pfn_valid(pfn)) { >>>>>>> + /* >>>>>>> + * VM will be able to see the page's tags, so we must ensure >>>>>>> + * they have been initialised. if PG_mte_tagged is set, tags >>>>>>> + * have already been initialised. >>>>>>> + */ >>>>>>> + struct page *page = pfn_to_page(pfn); >>>>>>> + unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT; >>>>>>> + >>>>>>> + for (i = 0; i < nr_pages; i++, page++) { >>>>>>> + if (!test_and_set_bit(PG_mte_tagged, &page->flags)) >>>>>>> + mte_clear_page_tags(page_address(page)); >>>>>>> + } >>>>>>> + } >>>>>> >>>>>> This pfn_valid() check may be problematic. Following commit eeb0753ba27b >>>>>> ("arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory"), it returns >>>>>> true for ZONE_DEVICE memory but such memory is allowed not to support >>>>>> MTE. >>>>> >>>>> Some more thinking, this should be safe as any ZONE_DEVICE would be >>>>> mapped as untagged memory in the kernel linear map. It could be slightly >>>>> inefficient if it unnecessarily tries to clear tags in ZONE_DEVICE, >>>>> untagged memory. Another overhead is pfn_valid() which will likely end >>>>> up calling memblock_is_map_memory(). >>>>> >>>>> However, the bigger issue is that Stage 2 cannot disable tagging for >>>>> Stage 1 unless the memory is Non-cacheable or Device at S2. Is there a >>>>> way to detect what gets mapped in the guest as Normal Cacheable memory >>>>> and make sure it's only early memory or hotplug but no ZONE_DEVICE (or >>>>> something else like on-chip memory)? If we can't guarantee that all >>>>> Cacheable memory given to a guest supports tags, we should disable the >>>>> feature altogether. >>>> >>>> In stage 2 I believe we only have two types of mapping - 'normal' or >>>> DEVICE_nGnRE (see stage2_map_set_prot_attr()). Filtering out the latter is a >>>> case of checking the 'device' variable, and makes sense to avoid the >>>> overhead you describe. >>>> >>>> This should also guarantee that all stage-2 cacheable memory supports tags, >>>> as kvm_is_device_pfn() is simply !pfn_valid(), and pfn_valid() should only >>>> be true for memory that Linux considers "normal". >> >> If you think "normal" == "normal System RAM", that's wrong; see below. > > By "normal" I think both Steven and I meant the Normal Cacheable memory > attribute (another being the Device memory attribute). > >>> That's the problem. With Anshuman's commit I mentioned above, >>> pfn_valid() returns true for ZONE_DEVICE mappings (e.g. persistent >>> memory, not talking about some I/O mapping that requires Device_nGnRE). >>> So kvm_is_device_pfn() is false for such memory and it may be mapped as >>> Normal but it is not guaranteed to support tagging. >> >> pfn_valid() means "there is a struct page"; if you do pfn_to_page() and >> touch the page, you won't fault. So Anshuman's commit is correct. > > I agree. > >> pfn_to_online_page() means, "there is a struct page and it's system RAM >> that's in use; the memmap has a sane content" > > Does pfn_to_online_page() returns a valid struct page pointer for > ZONE_DEVICE pages? IIUC, these are not guaranteed to be system RAM, for > some definition of system RAM (I assume NVDIMM != system RAM). For > example, pmem_attach_disk() calls devm_memremap_pages() and this would > use the Normal Cacheable memory attribute without necessarily being > system RAM.
No, not for ZONE_DEVICE.
However, if you expose PMEM via dax/kmem as System RAM to the system (-> add_memory_driver_managed()), then PMEM (managed via ZONE_NOMRAL or ZONE_MOVABLE) would work with pfn_to_online_page() -- as the system thinks it's "ordinary system RAM" and the memory is managed by the buddy.
> > So if pfn_valid() is not equivalent to system RAM, we have a potential > issue with MTE. Even if "system RAM" includes NVDIMMs, we still have > this issue and we may need a new term to describe MTE-safe memory. In > the kernel we assume MTE-safe all pages that can be mapped as > MAP_ANONYMOUS and I don't think these include ZONE_DEVICE pages. > > Thanks. >
-- Thanks,
David / dhildenb
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