Re: [PATCH v4 28/34] KVM: arm64: Use page-table to track page ownership

[Quentin Perret <qperret@google.com>]

On Friday 12 Mar 2021 at 09:32:06 (+0000), Will Deacon wrote:
> I'm not saying to use the VMID directly, just that allocating half of the
> pte feels a bit OTT given that the state of things after this patch series
> is that we're using exactly 1 bit.

Right, and that was the reason for the PROT_NONE approach in the
previous version, but we agreed it'd be worth generalizing to allow for
future use-cases :-)

> > > > @@ -517,28 +543,36 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > > >  	if (!kvm_block_mapping_supported(addr, end, phys, level))
> > > >  		return -E2BIG;
> > > >  
> > > > -	new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> > > > -	if (kvm_pte_valid(old)) {
> > > > +	if (kvm_pte_valid(data->attr))
> > > 
> > > This feels like a bit of a hack to me: the 'attr' field in stage2_map_data
> > > is intended to correspond directly to the lower/upper attributes of the
> > > descriptor as per the architecture, so tagging the valid bit in there is
> > > pretty grotty. However, I can see the significant advantage in being able
> > > to re-use the stage2_map_walker functionality, so about instead of nobbling
> > > attr, you set phys to something invalid instead, e.g.:
> > > 
> > > 	#define KVM_PHYS_SET_OWNER	(-1ULL)
> > 
> > That'll confuse kvm_block_mapping_supported() and friends I think, at
> > least in their current form. If you _really_ don't like this, maybe we
> > could have an extra 'flags' field in stage2_map_data?
> 
> I was pondering this last night and I thought of two ways to do it:
> 
> 1. Add a 'bool valid' and then stick the owner and the phys in a union.
>    (yes, you'll need to update the block mapping checks to look at the
>     valid flag)

Right, though that is also used for the hyp s1 which doesn't use any of
this ATM. That shouldn't be too bad to change, I'll have a look.

> 2. Go with my latter suggestion:
> 
> > > Is there ever a reason to use kvm_pgtable_stage2_set_owner() to set an
> > > owner of 0, or should you just use the map/unmap APIs for that? If so,
> > > then maybe the key is simply if owner_id is non-zero, then an invalid
> > > entry is installed?
> > 
> > I couldn't find a good reason to restrict it, as that wouldn't change
> > the implementation much anyway. Also, if we added the right CMOs, we
> > could probably remove the unmap walker and re-express it in terms of
> > set_owner(0) ... But I suppose that is for later :-)
> 
> The idea being that if owner is 0, then we install a mapping for phys, but
> if owner is !0 then we set the invalid mapping.

And I could even special-case set_owner(0) by calling unmap under the
hood so the caller doesn't need to care, but it's a bit yuck.

> (1) is probably the less hacky option... what do you reckon?

Agreed, (1) is a bit nicer. I was also considering setting phys = BIT(63)
in the set_owner() path. That makes it obvious it is an invalid PA, and
it should retain the nice alignment properties I need. But I suppose an
explicit flag makes it easier to reason about, so I'll have a go at it.

Thanks,
Quentin
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