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

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Message-ID: <e0b88560-34e1-dcc4-aaa7-9a7a5b771824@arm.com>
Date:   Mon, 29 Mar 2021 17:06:51 +0100
From:   Steven Price <steven.price@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature

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".

>> I now wonder if we can get a MAP_ANONYMOUS mapping of ZONE_DEVICE pfn
>> even without virtualisation.
> 
> I haven't checked all the code paths but I don't think we can get a
> MAP_ANONYMOUS mapping of ZONE_DEVICE memory as we normally need a file
> descriptor.
> 

I certainly hope this is the case - it's the weird corner cases of 
device drivers that worry me. E.g. I know i915 has a "hidden" mmap 
behind an ioctl (see i915_gem_mmap_ioctl(), although this case is fine - 
it's MAP_SHARED). Mali's kbase did something similar in the past.

Steve