Hi Marc,
On 2/25/21 9:55 AM, Marc Zyngier wrote:
> Hi Yanan,
>
> On Mon, 08 Feb 2021 11:22:47 +0000,
> Yanan Wang <wangyanan55@xxxxxxxxxx> 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?
Wouldn't that introduce overhead for the common case, when there's only one VCPU
that faults on an address? For each data abort caused by a missing stage 2 entry
we would now have to determine if the IPA isn't already mapped and that means
walking the stage 2 tables.
Or am I mistaken and either:
(a) The common case is multiple simultaneous translation faults from different
VCPUs on the same IPA. Or
(b) There's a fast way to check if an IPA is mapped at stage 2 and the overhead
would be negligible.
>
> Can you quantify the benefit of this patch alone?
>
>> Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
>> ---
>> 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.
I noticed yet another thing which I don't understand. When the CPU has the
ARM64_HAS_CACHE_DIC featue (CTR_EL0.DIC = 1), which means instruction invalidation
is not required for data to instruction coherence, we still do the icache
invalidation. I am wondering if the invalidation is necessary in this case.
If it's not, then I think it's correct (and straightforward) to move the icache
invalidation to stage2_map_walker_try_leaf() after the dcache clean+inval and make
it depend on the new mapping being executable *and*
!cpus_have_const_cap(ARM64_HAS_CACHE_DIC).
If the icache invalidation is required even if ARM64_HAS_CACHE_DIC is present,
then I'm not sure how we can distinguish between setting the executable
permissions because exec_fault (the code above) and setting the same permissions
because cpus_have_const_cap(ARM64_HAS_CACHE_DIC) (the code immediately following
the snippet above).
Thanks,
Alex
>
> Thanks,
>
> M.
>
