Re: [PATCH 02/16] KVM: arm64: Don't issue CMOs when the physical address is invalid

[Marc Zyngier]

On Mon, 19 Jul 2021 18:18:02 +0100,
Quentin Perret <qperret@xxxxxxxxxx> wrote:
> 
> On Thursday 15 Jul 2021 at 17:31:45 (+0100), Marc Zyngier wrote:
> > Make sure we don't issue CMOs when mapping something that
> > is not a memory address in the S2 page tables.
> > 
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 05321f4165e3..a5874ebd0354 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -619,12 +619,16 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> >  	}
> >  
> >  	/* Perform CMOs before installation of the guest stage-2 PTE */
> > -	if (mm_ops->dcache_clean_inval_poc && stage2_pte_cacheable(pgt, new))
> > -		mm_ops->dcache_clean_inval_poc(kvm_pte_follow(new, mm_ops),
> > -						granule);
> > -
> > -	if (mm_ops->icache_inval_pou && stage2_pte_executable(new))
> > -		mm_ops->icache_inval_pou(kvm_pte_follow(new, mm_ops), granule);
> > +	if (kvm_phys_is_valid(phys)) {
> > +		if (mm_ops->dcache_clean_inval_poc &&
> > +		    stage2_pte_cacheable(pgt, new))
> > +			mm_ops->dcache_clean_inval_poc(kvm_pte_follow(new,
> > +								      mm_ops),
> > +						       granule);
> > +		if (mm_ops->icache_inval_pou && stage2_pte_executable(new))
> > +			mm_ops->icache_inval_pou(kvm_pte_follow(new, mm_ops),
> > +						 granule);
> > +	}
> 
> Hrmpf so this makes me realize we have a problem here, not really caused
> by your patch though.
> 
> Specifically, calling kvm_pgtable_stage2_set_owner() can lead to
> overriding valid mappings with invalid mappings, which is effectively an
> unmap operation. In this case we should issue CMOs when unmapping a
> cacheable page to ensure it is clean to the PoC, like the
> kvm_pgtable_stage2_unmap() does.

You only need that if you to have a non-cacheable mapping to the same
page. Otherwise, you will be fine.

For pages that are moved from host-EL1 to EL2, we're good (I don't
think we ever have a non-cachable mapping at EL2). However, once we
start moving pages to guests, we'll need something.

> Note that you patch is already an improvement over the current state of
> things, because calling stage2_pte_cacheable(pgt, new),
> kvm_pte_follow(new, mm_ops) and friends is bogus when 'new' is invalid

Yeah, I had it mentally earmarked as a potential stable candidate. We
may have to do a bit better in the light of the above though.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.