linux-kernel - Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler

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Message-ID: <9a3bd48c4e69946bc4ade274ce2cc318@kernel.org>
Date:   Tue, 15 Dec 2020 13:18:35 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     "wangyanan (Y)" <wangyanan55@...wei.com>
Cc:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Gavin Shan <gshan@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        wanghaibin.wang@...wei.com, yezengruan@...wei.com,
        zhukeqian1@...wei.com, yuzenghui@...wei.com,
        jiangkunkun@...wei.com, wangjingyi11@...wei.com,
        lushenming@...wei.com
Subject: Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions
 only case in stage2 translation fault handler

Hi Yanan,

On 2020-12-14 07:20, wangyanan (Y) wrote:

> diff --git a/arch/arm64/kvm/hyp/pgtable.c 
> b/arch/arm64/kvm/hyp/pgtable.c
> index a74a62283012..e3c6133567c4 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -45,6 +45,10 @@
> 
>  #define KVM_PTE_LEAF_ATTR_HI_S2_XN     BIT(54)
> 
> +#define KVM_PTE_LEAF_ATTR_S2_PERMS (KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R | \
> +        KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
> + KVM_PTE_LEAF_ATTR_HI_S2_XN)
> +
>  struct kvm_pgtable_walk_data {
>         struct kvm_pgtable                   *pgt;
>         struct kvm_pgtable_walker       *walker;
> @@ -473,8 +477,13 @@ static bool stage2_map_walker_try_leaf(u64 addr,
> u64 end, u32 level,
> 
>         new = kvm_init_valid_leaf_pte(phys, data->attr, level);
>         if (kvm_pte_valid(old)) {
> -               /* Tolerate KVM recreating the exact same mapping. */
> -               if (old == new)
> +               /*
> +                * Skip updating the PTE with break-before-make if we 
> are trying
> +                * to recreate the exact same mapping or only change 
> the access
> +                * permissions. Actually, change of permissions will be 
> handled
> +                * through the relax_perms path next time if necessary.
> +                */
> +               if (!((old ^ new) & (~KVM_PTE_LEAF_ATTR_S2_PERMS)))
>                         goto out;
> 
>                 /* There's an existing different valid leaf entry, so 
> perform

I think there is a bit more work to do on this.

One obvious issue is that we currently flag a page as dirty before 
handling
the fault. With an early exit, we end-up having spurious dirty pages.

It's not a big deal, but I'd rather mark the page dirty after the 
mapping
or the permission update having been successful (at the moment, it 
cannot
fails).

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...