Messages in this thread | | | Subject | Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature | From | David Hildenbrand <> | Date | Thu, 8 Apr 2021 20:16:17 +0200 |
| |
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
| |