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

From: David Hildenbrand <david@redhat.com>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Steven Price <steven.price@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	Andrew Jones <drjones@redhat.com>, Haibo Xu <Haibo.Xu@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Juan Quintela <quintela@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	qemu-devel@nongnu.org, James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu,
	Thomas Gleixner <tglx@linutronix.de>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Will Deacon <will@kernel.org>, Dave Martin <Dave.Martin@arm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature
Date: Thu, 8 Apr 2021 20:16:17 +0200	[thread overview]
Message-ID: <bfcd1c41-92fb-d4ee-34b1-7beb6b6c9fd8@redhat.com> (raw)
In-Reply-To: <20210408141853.GA7676@arm.com>

On 08.04.21 16:18, Catalin Marinas wrote:
> On Wed, Apr 07, 2021 at 04:52:54PM +0100, Steven Price wrote:
>> On 07/04/2021 16:14, Catalin Marinas wrote:
>>> On Wed, Apr 07, 2021 at 11:20:18AM +0100, Steven Price wrote:
>>>> On 31/03/2021 19:43, Catalin Marinas wrote:
>>>>> When a slot is added by the VMM, if it asked for MTE in guest (I guess
>>>>> that's an opt-in by the VMM, haven't checked the other patches), can we
>>>>> reject it if it's is going to be mapped as Normal Cacheable but it is a
>>>>> ZONE_DEVICE (i.e. !kvm_is_device_pfn() + one of David's suggestions to
>>>>> check for ZONE_DEVICE)? This way we don't need to do more expensive
>>>>> checks in set_pte_at().
>>>>
>>>> The problem is that KVM allows the VMM to change the memory backing a slot
>>>> while the guest is running. This is obviously useful for the likes of
>>>> migration, but ultimately means that even if you were to do checks at the
>>>> time of slot creation, you would need to repeat the checks at set_pte_at()
>>>> time to ensure a mischievous VMM didn't swap the page for a problematic one.
>>>
>>> Does changing the slot require some KVM API call? Can we intercept it
>>> and do the checks there?
>>
>> As David has already replied - KVM uses MMU notifiers, so there's not really
>> a good place to intercept this before the fault.
>>
>>> Maybe a better alternative for the time being is to add a new
>>> kvm_is_zone_device_pfn() and force KVM_PGTABLE_PROT_DEVICE if it returns
>>> true _and_ the VMM asked for MTE in guest. We can then only set
>>> PG_mte_tagged if !device.
>>
>> KVM already has a kvm_is_device_pfn(), and yes I agree restricting the MTE
>> checks to only !kvm_is_device_pfn() makes sense (I have the fix in my branch
>> locally).
> 
> Indeed, you can skip it if kvm_is_device_pfn(). In addition, with MTE,
> I'd also mark a pfn as 'device' in user_mem_abort() if
> pfn_to_online_page() is NULL as we don't want to map it as Cacheable in
> Stage 2. It's unlikely that we'll trip over this path but just in case.
> 
> (can we have a ZONE_DEVICE _online_ pfn or by definition they are
> considered offline?)

By definition (and implementation) offline. When you get a page = 
pfn_to_online_page() with page != NULL, that one should never be 
ZONE_DEVICE (otherwise it would be a BUG).

As I said, things are different when exposing dax memory via dax/kmem to 
the buddy. But then, we are no longer talking about ZONE_DEVICE.

-- 
Thanks,

David / dhildenb