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Message-ID: <20201211095337.GA11280@willie-the-truck>
Date: Fri, 11 Dec 2020 09:53:37 +0000
From: Will Deacon <will@...nel.org>
To: Yanan Wang <wangyanan55@...wei.com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
Marc Zyngier <maz@...nel.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 Fri, Dec 11, 2020 at 04:01:15PM +0800, Yanan Wang wrote:
> In dirty-logging, or dirty-logging-stopped time, even normal running
> time of a guest configed with huge mappings and numbers of vCPUs,
> translation faults by different vCPUs on the same GPA could occur
> successively almost at the same time. There are two reasons for it.
>
> (1) If there are some vCPUs accessing the same GPA at the same time
> and the leaf PTE is not set yet, then they will all cause translation
> faults and the first vCPU holding mmu_lock will set valid leaf PTE,
> and the others will later choose to update the leaf PTE or not.
>
> (2) When changing a leaf entry or a table entry with break-before-make,
> if there are some vCPUs accessing the same GPA just catch the moment
> when the target PTE is set invalid in a BBM procedure coincidentally,
> they will all cause translation faults and will later choose to update
> the leaf PTE or not.
>
> The worst case can be like this: some vCPUs cause translation faults
> on the same GPA with different prots, they will fight each other by
> changing back access permissions of the PTE with break-before-make.
> And the BBM-invalid moment might trigger more unnecessary translation
> faults. As a result, some useless small loops will occur, which could
> lead to vCPU stuck.
>
> To avoid unnecessary update and small loops, add prejudgement in the
> translation fault handler: Skip updating the valid leaf PTE if we are
> trying to recreate exactly the same mapping or to reduce access
> permissions only(such as RW-->RO). And update the valid leaf PTE without
> break-before-make if we are trying to add more permissions only.
>
> Signed-off-by: Yanan Wang <wangyanan55@...wei.com>
> ---
> arch/arm64/kvm/hyp/pgtable.c | 73 +++++++++++++++++++++++++-----------
> 1 file changed, 52 insertions(+), 21 deletions(-)
Cheers for this. Given that this patch is solving a few different problems,
do you think you could split it up please? That would certainly make it much
easier to review, as there's quite a lot going on here. A chunk of the
changes seem to be the diff I posted previously:
https://lore.kernel.org/r/20201201141632.GC26973@willie-the-truck
so maybe that could be its own patch?
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 23a01dfcb27a..f8b3248cef1c 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -45,6 +45,8 @@
>
> #define KVM_PTE_LEAF_ATTR_HI_S2_XN BIT(54)
>
> +#define KVM_PTE_LEAF_ATTR_PERMS (GENMASK(7, 6) | BIT(54))
You only use this on the S2 path, so how about:
#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
or something like that?
> struct kvm_pgtable_walk_data {
> struct kvm_pgtable *pgt;
> struct kvm_pgtable_walker *walker;
> @@ -170,10 +172,9 @@ static void kvm_set_table_pte(kvm_pte_t *ptep, kvm_pte_t *childp)
> smp_store_release(ptep, pte);
> }
>
> -static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
> - u32 level)
> +static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
> {
> - kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(pa);
> + kvm_pte_t pte = kvm_phys_to_pte(pa);
> u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? KVM_PTE_TYPE_PAGE :
> KVM_PTE_TYPE_BLOCK;
>
> @@ -181,12 +182,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
> pte |= FIELD_PREP(KVM_PTE_TYPE, type);
> pte |= KVM_PTE_VALID;
>
> - /* Tolerate KVM recreating the exact same mapping. */
> - if (kvm_pte_valid(old))
> - return old == pte;
> -
> - smp_store_release(ptep, pte);
> - return true;
> + return pte;
> }
>
> static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
> @@ -341,12 +337,17 @@ static int hyp_map_set_prot_attr(enum kvm_pgtable_prot prot,
> static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> kvm_pte_t *ptep, struct hyp_map_data *data)
> {
> + kvm_pte_t new, old = *ptep;
> u64 granule = kvm_granule_size(level), phys = data->phys;
>
> if (!kvm_block_mapping_supported(addr, end, phys, level))
> return false;
>
> - WARN_ON(!kvm_set_valid_leaf_pte(ptep, phys, data->attr, level));
> + /* Tolerate KVM recreating the exact same mapping. */
> + new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> + if (old != new && !WARN_ON(kvm_pte_valid(old)))
> + smp_store_release(ptep, new);
> +
> data->phys += granule;
> return true;
> }
> @@ -461,25 +462,56 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
> return 0;
> }
>
> +static bool stage2_set_valid_leaf_pte_pre(u64 addr, u32 level,
> + kvm_pte_t *ptep, kvm_pte_t new,
> + struct stage2_map_data *data)
> +{
> + kvm_pte_t old = *ptep, old_attr, new_attr;
> +
> + if ((old ^ new) & (~KVM_PTE_LEAF_ATTR_PERMS))
> + return false;
> +
> + /*
> + * Skip updating if we are trying to recreate exactly the same mapping
> + * or to reduce the access permissions only. And update the valid leaf
> + * PTE without break-before-make if we are trying to add more access
> + * permissions only.
> + */
> + old_attr = (old & KVM_PTE_LEAF_ATTR_PERMS) ^ KVM_PTE_LEAF_ATTR_HI_S2_XN;
> + new_attr = (new & KVM_PTE_LEAF_ATTR_PERMS) ^ KVM_PTE_LEAF_ATTR_HI_S2_XN;
> + if (new_attr <= old_attr)
> + return true;
I think this is a significant change in behaviour for
kvm_pgtable_stage2_map() and I worry that it could catch somebody out in the
future. Please can you update the kerneldoc in kvm_pgtable.h with a note
about this?
Will
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