[RFC PATCH 1/4] KVM: arm64: Move the clean of dcache to the map handler

[Marc Zyngier]

Hi Yanan,

On Mon, 08 Feb 2021 11:22:47 +0000,
Yanan Wang <wangyanan55 at huawei.com> wrote:
> 
> We currently uniformly clean dcache in user_mem_abort() before calling the
> fault handlers, if we take a translation fault and the pfn is cacheable.
> But if there are concurrent translation faults on the same page or block,
> clean of dcache for the first time is necessary while the others are not.
> 
> By moving clean of dcache to the map handler, we can easily identify the
> conditions where CMOs are really needed and avoid the unnecessary ones.
> As it's a time consuming process to perform CMOs especially when flushing
> a block range, so this solution reduces much load of kvm and improve the
> efficiency of creating mappings.

That's an interesting approach. However, wouldn't it be better to
identify early that there is already something mapped, and return to
the guest ASAP?

Can you quantify the benefit of this patch alone?

> 
> Signed-off-by: Yanan Wang <wangyanan55 at huawei.com>
> ---
>  arch/arm64/include/asm/kvm_mmu.h | 16 --------------
>  arch/arm64/kvm/hyp/pgtable.c     | 38 ++++++++++++++++++++------------
>  arch/arm64/kvm/mmu.c             | 14 +++---------
>  3 files changed, 27 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index e52d82aeadca..4ec9879e82ed 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -204,22 +204,6 @@ static inline bool vcpu_has_cache_enabled(struct kvm_vcpu *vcpu)
>  	return (vcpu_read_sys_reg(vcpu, SCTLR_EL1) & 0b101) == 0b101;
>  }
>  
> -static inline void __clean_dcache_guest_page(kvm_pfn_t pfn, unsigned long size)
> -{
> -	void *va = page_address(pfn_to_page(pfn));
> -
> -	/*
> -	 * With FWB, we ensure that the guest always accesses memory using
> -	 * cacheable attributes, and we don't have to clean to PoC when
> -	 * faulting in pages. Furthermore, FWB implies IDC, so cleaning to
> -	 * PoU is not required either in this case.
> -	 */
> -	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> -		return;
> -
> -	kvm_flush_dcache_to_poc(va, size);
> -}
> -
>  static inline void __invalidate_icache_guest_page(kvm_pfn_t pfn,
>  						  unsigned long size)
>  {
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 4d177ce1d536..2f4f87021980 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -464,6 +464,26 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
>  	return 0;
>  }
>  
> +static bool stage2_pte_cacheable(kvm_pte_t pte)
> +{
> +	u64 memattr = pte & KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR;
> +	return memattr == PAGE_S2_MEMATTR(NORMAL);
> +}
> +
> +static void stage2_flush_dcache(void *addr, u64 size)
> +{
> +	/*
> +	 * With FWB, we ensure that the guest always accesses memory using
> +	 * cacheable attributes, and we don't have to clean to PoC when
> +	 * faulting in pages. Furthermore, FWB implies IDC, so cleaning to
> +	 * PoU is not required either in this case.
> +	 */
> +	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> +		return;
> +
> +	__flush_dcache_area(addr, size);
> +}
> +
>  static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  				      kvm_pte_t *ptep,
>  				      struct stage2_map_data *data)
> @@ -495,6 +515,10 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  		put_page(page);
>  	}
>  
> +	/* Flush data cache before installation of the new PTE */
> +	if (stage2_pte_cacheable(new))
> +		stage2_flush_dcache(__va(phys), granule);
> +
>  	smp_store_release(ptep, new);
>  	get_page(page);
>  	data->phys += granule;
> @@ -651,20 +675,6 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  	return ret;
>  }
>  
> -static void stage2_flush_dcache(void *addr, u64 size)
> -{
> -	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> -		return;
> -
> -	__flush_dcache_area(addr, size);
> -}
> -
> -static bool stage2_pte_cacheable(kvm_pte_t pte)
> -{
> -	u64 memattr = pte & KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR;
> -	return memattr == PAGE_S2_MEMATTR(NORMAL);
> -}
> -
>  static int stage2_unmap_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>  			       enum kvm_pgtable_walk_flags flag,
>  			       void * const arg)
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 77cb2d28f2a4..d151927a7d62 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -609,11 +609,6 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>  	kvm_mmu_write_protect_pt_masked(kvm, slot, gfn_offset, mask);
>  }
>  
> -static void clean_dcache_guest_page(kvm_pfn_t pfn, unsigned long size)
> -{
> -	__clean_dcache_guest_page(pfn, size);
> -}
> -
>  static void invalidate_icache_guest_page(kvm_pfn_t pfn, unsigned long size)
>  {
>  	__invalidate_icache_guest_page(pfn, size);
> @@ -882,9 +877,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	if (writable)
>  		prot |= KVM_PGTABLE_PROT_W;
>  
> -	if (fault_status != FSC_PERM && !device)
> -		clean_dcache_guest_page(pfn, vma_pagesize);
> -
>  	if (exec_fault) {
>  		prot |= KVM_PGTABLE_PROT_X;
>  		invalidate_icache_guest_page(pfn, vma_pagesize);

It seems that the I-side CMO now happens *before* the D-side, which
seems odd. What prevents the CPU from speculatively fetching
instructions in the interval? I would also feel much more confident if
the two were kept close together.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.