[PATCH REPOST 3/3] arm64: kvm: Fix single step for guest skipped instructions

[Alex Bennée]

Julien Thierry <julien.thierry at arm.com> writes:

> Software Step exception is missing after trapping instruction from the guest.
>
> We need to set the PSR.SS to 0 for the guest vcpu before resuming guest
> execution.
>
> Signed-off-by: Julien Thierry <julien.thierry at arm.com>
> Cc: Christoffer Dall <christoffer.dall at linaro.org>
> Cc: Marc Zyngier <marc.zyngier at arm.com>
> Cc: Alex Bennée <alex.bennee at linaro.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will.deacon at arm.com>
>
> ---
>  arch/arm64/include/asm/kvm_asm.h     |  2 ++
>  arch/arm64/include/asm/kvm_emulate.h |  2 ++
>  arch/arm64/kvm/debug.c               | 17 ++++++++++++++++-
>  arch/arm64/kvm/hyp/switch.c          | 10 ++++++++++
>  4 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 26a64d0..398bbaa 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -32,6 +32,8 @@
>
>  #define KVM_ARM64_DEBUG_DIRTY_SHIFT	0
>  #define KVM_ARM64_DEBUG_DIRTY		(1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)
> +#define KVM_ARM64_DEBUG_INST_SKIP_SHIFT	1
> +#define KVM_ARM64_DEBUG_INST_SKIP	(1 << KVM_ARM64_DEBUG_INST_SKIP_SHIFT)
>
>  #define kvm_ksym_ref(sym)						\
>  	({								\
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index e5df3fc..ee02dd2ee 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -95,6 +95,8 @@ static inline void kvm_skip_instr(struct kvm_vcpu *vcpu, bool is_wide_instr)
>  		kvm_skip_instr32(vcpu, is_wide_instr);
>  	else
>  		*vcpu_pc(vcpu) += 4;
> +	/* Let debug engine know we skipped an instruction */
> +	vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_INST_SKIP;
>  }

I can see the appeal of handling things here but I think for both
trapped system instructions and mmio emulation everything flows back to
handle_exit() where we could deal with it there and then.

>
>  static inline void vcpu_set_thumb(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index dbadfaf..b5fcb96 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -151,12 +151,27 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  		 * debugging the system.
>  		 */
>  		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> -			*vcpu_cpsr(vcpu) |=  DBG_SPSR_SS;
> +			/*
> +			 * Taking a Software step exception, context being
> +			 * stepped has PSTATE.SS == 0. In order to step the next
> +			 * instruction, we need to reset this bit.
> +			 * If we skipped an instruction while single stepping,
> +			 * we want to get a software step exception for the
> +			 * skipped instruction (i.e. as soon as we return to the
> +			 * guest). This is obtained by returning to the guest
> +			 * with PSTATE.SS cleared.
> +			 */
> +			if (!(vcpu->arch.debug_flags & KVM_ARM64_DEBUG_INST_SKIP))
> +				*vcpu_cpsr(vcpu) |=  DBG_SPSR_SS;
> +			else
> +				*vcpu_cpsr(vcpu) &=  ~DBG_SPSR_SS;
>  			vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_SS;
>  		} else {
>  			vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
>  		}
>
> +		vcpu->arch.debug_flags &= ~KVM_ARM64_DEBUG_INST_SKIP;
> +
>  		trace_kvm_arm_set_dreg32("SPSR_EL2", *vcpu_cpsr(vcpu));
>
>  		/*
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 945e79c..34fe215 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -22,6 +22,7 @@
>  #include <asm/kvm_emulate.h>
>  #include <asm/kvm_hyp.h>
>  #include <asm/fpsimd.h>
> +#include <asm/debug-monitors.h>
>
>  static bool __hyp_text __fpsimd_enabled_nvhe(void)
>  {
> @@ -276,6 +277,8 @@ static void __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
>  	}
>
>  	write_sysreg_el2(*vcpu_pc(vcpu), elr);
> +
> +	write_sysreg_el2(read_sysreg_el2(spsr) & ~DBG_SPSR_SS, spsr);
>  }

Hmm this function seems a bit magical - given it is manipulating PC/ELR.
I guess it is only called for eret and other exception returns? I wonder
if a rename to __skip_eret_instr would be in order for clarity?

>
>  int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> @@ -343,6 +346,13 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
>  			if (ret == -1) {
>  				/* Promote an illegal access to an SError */
>  				__skip_instr(vcpu);
> +
> +				/*
> +				 * We're not jumping back, let debug setup know
> +				 * we skipped an instruction.
> +				 */
> +				vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_INST_SKIP;
> +

I'm not so sure of this. We are exiting so it seems we are going to
persist some debug state over an exit where an ioctl may change things
before we re-enter.

>  				exit_code = ARM_EXCEPTION_EL1_SERROR;
>  			}


--
Alex Bennée