Re: [PATCH v3 08/15] KVM: arm64: Protect stage-2 traversal with RCU

From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	Reiji Watanabe <reijiw@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	David Matlack <dmatlack@google.com>,
	Quentin Perret <qperret@google.com>,
	Ben Gardon <bgardon@google.com>, Gavin Shan <gshan@redhat.com>,
	Peter Xu <peterx@redhat.com>, Will Deacon <will@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	kvmarm@lists.linux.dev
Subject: Re: [PATCH v3 08/15] KVM: arm64: Protect stage-2 traversal with RCU
Date: Fri, 28 Oct 2022 09:19:15 +0000	[thread overview]
Message-ID: <Y1uek8RygHO+cXCF@google.com> (raw)
In-Reply-To: <20221027221752.1683510-9-oliver.upton@linux.dev>

On Thu, Oct 27, 2022 at 10:17:45PM +0000, Oliver Upton wrote:
> The use of RCU is necessary to safely change the stage-2 page tables in
> parallel. Acquire and release the RCU read lock when traversing the page
> tables.
> 
> Use the _raw() flavor of rcu_dereference when changes to the page tables
> are otherwise protected from parallel software walkers (e.g. holding the
> write lock).
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 41 ++++++++++++++++++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 10 ++++++-
>  2 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index e70cf57b719e..d1859e8550df 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -37,6 +37,13 @@ static inline u64 kvm_get_parange(u64 mmfr0)
>  
>  typedef u64 kvm_pte_t;
>  
> +/*
> + * RCU cannot be used in a non-kernel context such as the hyp. As such, page
> + * table walkers used in hyp do not call into RCU and instead use other
> + * synchronization mechanisms (such as a spinlock).
> + */
> +#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
> +
>  typedef kvm_pte_t *kvm_pteref_t;
>  
>  static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
> @@ -44,6 +51,40 @@ static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared
>  	return pteref;
>  }
>  
> +static inline void kvm_pgtable_walk_begin(void) {}
> +static inline void kvm_pgtable_walk_end(void) {}
> +
> +static inline bool kvm_pgtable_walk_lock_held(void)
> +{
> +	return true;
> +}
> +
> +#else
> +
> +typedef kvm_pte_t __rcu *kvm_pteref_t;
> +
> +static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
> +{
> +	return rcu_dereference_check(pteref, shared);

I accidentally squashed the fix for !shared into 9/15, not this patch.
Fix ready for v4.

--
Thanks,
Oliver

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