linux-kernel - Re: [PATCH 06/14] KVM: arm64: Return next table from map callbacks

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Date:   Fri, 9 Sep 2022 10:38:23 +0100
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     David Matlack <dmatlack@...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>,
        Will Deacon <will@...nel.org>,
        Quentin Perret <qperret@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Gavin Shan <gshan@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/14] KVM: arm64: Return next table from map callbacks

Hi David,

On Wed, Sep 07, 2022 at 02:32:29PM -0700, David Matlack wrote:
> On Tue, Aug 30, 2022 at 07:41:24PM +0000, Oliver Upton wrote:
> > The map walkers install new page tables during their traversal. Return
> > the newly-installed table PTE from the map callbacks to point the walker
> > at the new table w/o rereading the ptep.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@...ux.dev>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 331f6e3b2c20..f911509e6512 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -202,13 +202,12 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> >  	if (!table && (flags & KVM_PGTABLE_WALK_LEAF)) {
> >  		ret = kvm_pgtable_visitor_cb(data, addr, level, ptep, &pte,
> >  					     KVM_PGTABLE_WALK_LEAF);
> > -		pte = *ptep;
> > -		table = kvm_pte_table(pte, level);
> >  	}
> >  
> >  	if (ret)
> >  		goto out;
> 
> Rather than passing a pointer to the local variable pte and requiring
> all downstream code to update it (and deal with dereferencing to read
> the old pte), wouldn't it be simpler to just re-read the PTE here?

Yeah, you're right. I had some odd rationalization about this, but
there's no need to force a walker to descend into the new table level as
it is wasted work if another thread unlinks it.

[...]

> >  
> > +	table = kvm_pte_table(pte, level);
> >  	if (!table) {
> 
> nit: Technically there's no reason to set @table again. e.g. This could
> just be:
> 
>         if (!kvm_pte_table(pte, level)) {

Sure, I'll squish these lines together.

--
Thanks,
Oliver