linux-kernel - Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages

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Message-ID: <20211213125300.GA10915@willie-the-truck>
Date:   Mon, 13 Dec 2021 12:53:00 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages

On Fri, Dec 10, 2021 at 02:34:16PM +0000, Quentin Perret wrote:
> On Thursday 09 Dec 2021 at 10:29:24 (+0000), Will Deacon wrote:
> > On Wed, Dec 01, 2021 at 05:03:57PM +0000, Quentin Perret wrote:
> > > To prepare the ground for allowing hyp stage-1 mappings to be removed at
> > > run-time, update the KVM page-table code to maintain a correct refcount
> > > using the ->{get,put}_page() function callbacks.
> > > 
> > > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > > ---
> > >  arch/arm64/kvm/hyp/pgtable.c | 17 ++++++++++++++---
> > >  1 file changed, 14 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index f8ceebe4982e..768a58835153 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -408,8 +408,10 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > >  		return false;
> > >  
> > >  	new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> > > -	if (hyp_pte_needs_update(old, new))
> > > +	if (hyp_pte_needs_update(old, new)) {
> > >  		smp_store_release(ptep, new);
> > > +		data->mm_ops->get_page(ptep);
> > 
> > In the case where we're just updating software bits for a valid pte, doesn't
> > this result in us taking a spurious reference to the page?
> 
> Ahem, yes, that is the case. I ended up with the below diff to fix it,
> which I intend to fold in the next version:
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 6ad4cb2d6947..e2047d3f05a2 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -383,21 +383,6 @@ enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte)
>         return prot;
>  }
> 
> -static bool hyp_pte_needs_update(kvm_pte_t old, kvm_pte_t new)
> -{
> -       /*
> -        * Tolerate KVM recreating the exact same mapping, or changing software
> -        * bits if the existing mapping was valid.
> -        */
> -       if (old == new)
> -               return false;
> -
> -       if (!kvm_pte_valid(old))
> -               return true;
> -
> -       return !WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW);
> -}
> -
>  static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>                                     kvm_pte_t *ptep, struct hyp_map_data *data)
>  {
> @@ -407,13 +392,16 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>         if (!kvm_block_mapping_supported(addr, end, phys, level))
>                 return false;
> 
> +       data->phys += granule;
>         new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> -       if (hyp_pte_needs_update(old, new)) {
> -               smp_store_release(ptep, new);
> +       if (old == new)
> +               return true;
> +       else if (!kvm_pte_valid(old))

(nit: clearer to drop the 'else' part here)

>                 data->mm_ops->get_page(ptep);

Ok, so this works because new is always valid.

LGTM.

Will