Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature

[David Hildenbrand <david@redhat.com>]

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