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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On 2/24/21 5:39 PM, Marc Zyngier wrote:
> On Wed, 24 Feb 2021 17:21:22 +0000,
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
>> Hello,
>>
>> On 2/8/21 11:22 AM, Yanan Wang 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.
>>>
>>> Signed-off-by: Yanan Wang <wangyanan55@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);
>> This makes sense to me. kvm_pgtable_stage2_map() is protected
>> against concurrent calls by the kvm->mmu_lock, so only one VCPU can
>> change the stage 2 translation table at any given moment. In the
>> case of concurrent translation faults on the same IPA, the first
>> VCPU that will take the lock will create the mapping and do the
>> dcache clean+invalidate. The other VCPUs will return -EAGAIN because
>> the mapping they are trying to install is almost identical* to the
>> mapping created by the first VCPU that took the lock.
>>
>> I have a question. Why are you doing the cache maintenance *before*
>> installing the new mapping? This is what the kernel already does, so
>> I'm not saying it's incorrect, I'm just curious about the reason
>> behind it.
> The guarantee KVM offers to the guest is that by the time it can
> access the memory, it is cleaned to the PoC. If you establish a
> mapping before cleaning, another vcpu can access the PoC (no fault,
> you just set up S2) and not see it up to date.

Right, I knew I was missing something, thanks for the explanation.

Thanks,

Alex