Message-ID: <b688cf37-16e6-d068-d97f-146c64afca08@huawei.com>
Date:   Thu, 8 Apr 2021 17:23:11 +0800
From:   "wangyanan (Y)" <wangyanan55@...wei.com>
To:     Alexandru Elisei <alexandru.elisei@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     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>, <zhukeqian1@...wei.com>,
        <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v3 1/2] KVM: arm64: Move CMOs from user_mem_abort to
 the fault handlers

Hi Alex,

On 2021/4/7 23:31, Alexandru Elisei wrote:
> Hi Yanan,
>
> On 3/26/21 3:16 AM, Yanan Wang wrote:
>> We currently uniformly permorm CMOs of D-cache and I-cache in function
>> user_mem_abort before calling the fault handlers. If we get concurrent
>> guest faults(e.g. translation faults, permission faults) or some really
>> unnecessary guest faults caused by BBM, CMOs for the first vcpu are
> I can't figure out what BBM means.
Just as Will has explained, it's Break-Before-Make rule. When we need to
replace an old table entry with a new one, we should firstly invalidate
the old table entry(Break), before installation of the new entry(Make).

And I think this patch mainly introduces benefits in two specific scenarios:
1) In a VM startup, it will improve efficiency of handling page faults 
incurred
by vCPUs, when initially populating stage2 page tables.
2) After live migration, the heavy workload will be resumed on the 
destination
VMs, however all the stage2 page tables need to be rebuilt.
>> necessary while the others later are not.
>>
>> By moving CMOs to the fault handlers, we can easily identify conditions
>> where they 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 efficiency
>> of the page table code.
>>
>> So let's move both clean of D-cache and invalidation of I-cache to the
>> map path and move only invalidation of I-cache to the permission path.
>> Since the original APIs for CMOs in mmu.c are only called in function
>> user_mem_abort, we now also move them to pgtable.c.
>>
>> Signed-off-by: Yanan Wang <wangyanan55@...wei.com>
>> ---
>>   arch/arm64/include/asm/kvm_mmu.h | 31 ---------------
>>   arch/arm64/kvm/hyp/pgtable.c     | 68 +++++++++++++++++++++++++-------
>>   arch/arm64/kvm/mmu.c             | 23 ++---------
>>   3 files changed, 57 insertions(+), 65 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
>> index 90873851f677..c31f88306d4e 100644
>> --- a/arch/arm64/include/asm/kvm_mmu.h
>> +++ b/arch/arm64/include/asm/kvm_mmu.h
>> @@ -177,37 +177,6 @@ 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)
>> -{
>> -	if (icache_is_aliasing()) {
>> -		/* any kind of VIPT cache */
>> -		__flush_icache_all();
>> -	} else if (is_kernel_in_hyp_mode() || !icache_is_vpipt()) {
>> -		/* PIPT or VPIPT at EL2 (see comment in __kvm_tlb_flush_vmid_ipa) */
>> -		void *va = page_address(pfn_to_page(pfn));
>> -
>> -		invalidate_icache_range((unsigned long)va,
>> -					(unsigned long)va + size);
>> -	}
>> -}
>> -
>>   void kvm_set_way_flush(struct kvm_vcpu *vcpu);
>>   void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled);
>>   
>> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>> index 4d177ce1d536..829a34eea526 100644
>> --- a/arch/arm64/kvm/hyp/pgtable.c
>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>> @@ -464,6 +464,43 @@ 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 bool stage2_pte_executable(kvm_pte_t pte)
>> +{
>> +	return !(pte & KVM_PTE_LEAF_ATTR_HI_S2_XN);
>> +}
>> +
>> +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 void stage2_invalidate_icache(void *addr, u64 size)
>> +{
>> +	if (icache_is_aliasing()) {
>> +		/* Flush any kind of VIPT icache */
>> +		__flush_icache_all();
>> +	} else if (is_kernel_in_hyp_mode() || !icache_is_vpipt()) {
>> +		/* PIPT or VPIPT at EL2 */
>> +		invalidate_icache_range((unsigned long)addr,
>> +					(unsigned long)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 +532,13 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>>   		put_page(page);
>>   	}
>>   
>> +	/* Perform CMOs before installation of the new PTE */
>> +	if (!kvm_pte_valid(old) || stage2_pte_cacheable(old))
> I'm not sure why the stage2_pte_cacheable(old) condition is needed.
>
> kvm_handle_guest_abort() handles three types of stage 2 data or instruction
> aborts: translation faults (fault_status == FSC_FAULT), access faults
> (fault_status == FSC_ACCESS) and permission faults (fault_status == FSC_PERM).
>
> Access faults are handled in handle_access_fault(), which means user_mem_abort()
> handles translation and permission faults.
Yes, and we are certain that it's a translation fault here in 
stage2_map_walker_try_leaf.
> The original code did the dcache clean
> + inval when not a permission fault, which means the CMO was done only on a
> translation fault. Translation faults mean that the IPA was not mapped, so the old
> entry will always be invalid. Even if we're coalescing multiple last level leaf
> entries int oa  block mapping, the table entry which is replaced is invalid
> because it's marked as such in stage2_map_walk_table_pre().
>
> Is there something I'm missing?
I originally thought that we could possibly have a translation fault on 
a valid stage2 table
descriptor due to some special cases, and that's the reason 
stage2_pte_cacheable(old)
condition exits, but I can't image any scenario like this.

I think your above explanation is right, maybe I should just drop that 
condition.
>
>> +		stage2_flush_dcache(__va(phys), granule);
>> +
>> +	if (stage2_pte_executable(new))
>> +		stage2_invalidate_icache(__va(phys), granule);
> This, together with the stage2_attr_walker() changes below, look identical to the
> current code in user_mem_abort(). The executable permission is set on an exec
> fault (instruction abort not on a stage 2 translation table walk), and as a result
> of the fault we either need to map a new page here, or relax permissions in
> kvm_pgtable_stage2_relax_perms() -> stage2_attr_walker() below.
I agree.
Do you mean this part of change is right?

Thanks,
Yanan
> Thanks,
>
> Alex
>
>> +
>>   	smp_store_release(ptep, new);
>>   	get_page(page);
>>   	data->phys += granule;
>> @@ -651,20 +695,6 @@ 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)
>> @@ -743,8 +773,16 @@ static int stage2_attr_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>>   	 * but worst-case the access flag update gets lost and will be
>>   	 * set on the next access instead.
>>   	 */
>> -	if (data->pte != pte)
>> +	if (data->pte != pte) {
>> +		/*
>> +		 * Invalidate the instruction cache before updating
>> +		 * if we are going to add the executable permission.
>> +		 */
>> +		if (!stage2_pte_executable(*ptep) && stage2_pte_executable(pte))
>> +			stage2_invalidate_icache(kvm_pte_follow(pte),
>> +						 kvm_granule_size(level));
>>   		WRITE_ONCE(*ptep, pte);
>> +	}
>>   
>>   	return 0;
>>   }
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 77cb2d28f2a4..1eec9f63bc6f 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -609,16 +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);
>> -}
>> -
>>   static void kvm_send_hwpoison_signal(unsigned long address, short lsb)
>>   {
>>   	send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, lsb, current);
>> @@ -882,13 +872,8 @@ 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) {
>> +	if (exec_fault)
>>   		prot |= KVM_PGTABLE_PROT_X;
>> -		invalidate_icache_guest_page(pfn, vma_pagesize);
>> -	}
>>   
>>   	if (device)
>>   		prot |= KVM_PGTABLE_PROT_DEVICE;
>> @@ -1144,10 +1129,10 @@ int kvm_set_spte_hva(struct kvm *kvm, unsigned long hva, pte_t pte)
>>   	trace_kvm_set_spte_hva(hva);
>>   
>>   	/*
>> -	 * We've moved a page around, probably through CoW, so let's treat it
>> -	 * just like a translation fault and clean the cache to the PoC.
>> +	 * We've moved a page around, probably through CoW, so let's treat
>> +	 * it just like a translation fault and the map handler will clean
>> +	 * the cache to the PoC.
>>   	 */
>> -	clean_dcache_guest_page(pfn, PAGE_SIZE);
>>   	handle_hva_to_gpa(kvm, hva, end, &kvm_set_spte_handler, &pfn);
>>   	return 0;
>>   }
> .

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