linux-kernel - Re: [PATCH v3 10/15] KVM: arm64: Introduce wrappers for host and hyp spin lock accessors

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Date:   Tue, 14 Dec 2021 14:48:30 +0000
From:   Andrew Walbran <qwandor@...gle.com>
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>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 10/15] KVM: arm64: Introduce wrappers for host and hyp
 spin lock accessors

On Wed, 1 Dec 2021 at 17:04, 'Quentin Perret' via kernel-team
<kernel-team@...roid.com> wrote:
>
> From: Will Deacon <will@...nel.org>
>
> In preparation for adding additional locked sections for manipulating
> page-tables at EL2, introduce some simple wrappers around the host and
> hypervisor locks so that it's a bit easier to read and bit more difficult
> to take the wrong lock (or even take them in the wrong order).
Looks good, but how does this help prevent taking locks in the wrong order?

>
> Signed-off-by: Will Deacon <will@...nel.org>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 32 ++++++++++++++++++++++-----
>  1 file changed, 26 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index c1a90dd022b8..757dfefe3aeb 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -27,6 +27,26 @@ static struct hyp_pool host_s2_pool;
>
>  const u8 pkvm_hyp_id = 1;
>
> +static void host_lock_component(void)
> +{
> +       hyp_spin_lock(&host_kvm.lock);
> +}
> +
> +static void host_unlock_component(void)
> +{
> +       hyp_spin_unlock(&host_kvm.lock);
> +}
> +
> +static void hyp_lock_component(void)
> +{
> +       hyp_spin_lock(&pkvm_pgd_lock);
> +}
> +
> +static void hyp_unlock_component(void)
> +{
> +       hyp_spin_unlock(&pkvm_pgd_lock);
> +}
> +
>  static void *host_s2_zalloc_pages_exact(size_t size)
>  {
>         void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
> @@ -338,14 +358,14 @@ static int host_stage2_idmap(u64 addr)
>
>         prot = is_memory ? PKVM_HOST_MEM_PROT : PKVM_HOST_MMIO_PROT;
>
> -       hyp_spin_lock(&host_kvm.lock);
> +       host_lock_component();
>         ret = host_stage2_adjust_range(addr, &range);
>         if (ret)
>                 goto unlock;
>
>         ret = host_stage2_idmap_locked(range.start, range.end - range.start, prot);
>  unlock:
> -       hyp_spin_unlock(&host_kvm.lock);
> +       host_unlock_component();
>
>         return ret;
>  }
> @@ -369,8 +389,8 @@ int __pkvm_host_share_hyp(u64 pfn)
>         if (!addr_is_memory(addr))
>                 return -EINVAL;
>
> -       hyp_spin_lock(&host_kvm.lock);
> -       hyp_spin_lock(&pkvm_pgd_lock);
> +       host_lock_component();
> +       hyp_lock_component();
>
>         ret = kvm_pgtable_get_leaf(&host_kvm.pgt, addr, &pte, NULL);
>         if (ret)
> @@ -432,8 +452,8 @@ int __pkvm_host_share_hyp(u64 pfn)
>         BUG_ON(ret);
>
>  unlock:
> -       hyp_spin_unlock(&pkvm_pgd_lock);
> -       hyp_spin_unlock(&host_kvm.lock);
> +       hyp_unlock_component();
> +       host_unlock_component();
>
>         return ret;
>  }
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>
> --
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>