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

[Will Deacon]

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@xxxxxxxxxx>
> ---
>  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