Re: [RFC PATCH 3/3] KVM: arm64: Add usage of stage 2 fault lookup level in user_mem_abort()

wangyanan (Y)
2020-12-01T06:06:08Z

Hi Will,

On 2020/11/30 21:49, Will Deacon wrote:
> On Mon, Nov 30, 2020 at 08:18:47PM +0800, Yanan Wang wrote:
>> If we get a FSC_PERM fault, just using (logging_active && writable) to determine
>> calling kvm_pgtable_stage2_map(). There will be two more cases we should consider.
>>
>> (1) After logging_active is configged back to false from true. When we get a
>> FSC_PERM fault with write_fault and adjustment of hugepage is needed, we should
>> merge tables back to a block entry. This case is ignored by still calling
>> kvm_pgtable_stage2_relax_perms(), which will lead to an endless loop and guest
>> panic due to soft lockup.
>>
>> (2) We use (FSC_PERM && logging_active && writable) to determine collapsing
>> a block entry into a table by calling kvm_pgtable_stage2_map(). But sometimes
>> we may only need to relax permissions when trying to write to a page other than
>> a block. In this condition, using kvm_pgtable_stage2_relax_perms() will be fine.
>>
>> The ISS filed bit[1:0] in ESR_EL2 regesiter indicates the stage2 lookup level
>> at which a D-abort or I-abort occured. By comparing granule of the fault lookup
>> level with vma_pagesize, we can strictly distinguish conditions of calling
>> kvm_pgtable_stage2_relax_perms() or kvm_pgtable_stage2_map(), and the above
>> two cases will be well considered.
>>
>> Suggested-by: Keqian Zhu <zhukeqian1@huawei.com>
>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>> ---
>> arch/arm64/include/asm/esr.h | 1 +
>> arch/arm64/include/asm/kvm_emulate.h | 5 +++++
>> arch/arm64/kvm/mmu.c | 11 +++++++++--
>> 3 files changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
>> index 22c81f1edda2..85a3e49f92f4 100644
>> --- a/arch/arm64/include/asm/esr.h
>> +++ b/arch/arm64/include/asm/esr.h
>> @@ -104,6 +104,7 @@
>> /* Shared ISS fault status code(IFSC/DFSC) for Data/Instruction aborts */
>> #define ESR_ELx_FSC (0x3F)
>> #define ESR_ELx_FSC_TYPE (0x3C)
>> +#define ESR_ELx_FSC_LEVEL (0x03)
>> #define ESR_ELx_FSC_EXTABT (0x10)
>> #define ESR_ELx_FSC_SERROR (0x11)
>> #define ESR_ELx_FSC_ACCESS (0x08)
>> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
>> index 5ef2669ccd6c..2e0e8edf6306 100644
>> --- a/arch/arm64/include/asm/kvm_emulate.h
>> +++ b/arch/arm64/include/asm/kvm_emulate.h
>> @@ -350,6 +350,11 @@ static __always_inline u8 kvm_vcpu_trap_get_fault_type(const struct kvm_vcpu *vc
>> return kvm_vcpu_get_esr(vcpu) & ESR_ELx_FSC_TYPE;
>> }
>>
>> +static __always_inline u8 kvm_vcpu_trap_get_fault_level(const struct kvm_vcpu *vcpu)
>> +{
>> + return kvm_vcpu_get_esr(vcpu) & ESR_ELx_FSC_LEVEL;
>> +{
>> +
>> static __always_inline bool kvm_vcpu_abt_issea(const struct kvm_vcpu *vcpu)
>> {
>> switch (kvm_vcpu_trap_get_fault(vcpu)) {
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 1a01da9fdc99..75814a02d189 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -754,10 +754,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>> gfn_t gfn;
>> kvm_pfn_t pfn;
>> bool logging_active = memslot_is_logging(memslot);
>> - unsigned long vma_pagesize;
>> + unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
>> + unsigned long vma_pagesize, fault_granule;
>> enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
>> struct kvm_pgtable *pgt;
>>
>> + fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
> I like the idea, but is this macro reliable for stage-2 page-tables, given
> that we could have a concatenated pgd?
>
> Will
> .

Yes, it's fine even when we have a concatenated pgd table.

No matter a concatenated pgd will be made or not, the initial lookup
level (start _level) is set in VTCR_EL2 register.

The MMU hardware walker will know the start_level according to
information in VTCR_EL2.

This idea runs well in practice on host where ia_bits is 40, PAGE_SIZE
is 4k, and a concatenated pgd is made for guest stage2.

According to the kernel info printed, the start_level is 1, and stage 2
translation runs as expected.

Yanan