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

Will Deacon
2020-11-30T13:50:35Z

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