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

[Catalin Marinas <catalin.marinas@arm.com>]

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?)

> > BTW, after a page is restored from swap, how long do we keep the
> > metadata around? I think we can delete it as soon as it was restored and
> > PG_mte_tagged was set. Currently it looks like we only do this when the
> > actual page was freed or swapoff. I haven't convinced myself that it's
> > safe to do this for swapoff unless it guarantees that all the ptes
> > sharing a page have been restored.
> 
> My initial thought was to free the metadata immediately. However it turns
> out that the following sequence can happen:
> 
>  1. Swap out a page
>  2. Swap the page in *read only*
>  3. Discard the page
>  4. Swap the page in again
> 
> So there's no writing of the swap data again before (3). This works nicely
> with a swap device because after writing a page it stays there forever, so
> if you know it hasn't been modified it's pointless rewriting it. Sadly it's
> not quite so ideal with the MTE tags which are currently kept in RAM.

I missed this scenario. So we need to keep it around as long as the
corresponding swap storage is still valid.

-- 
Catalin