[PATCH v4 5/5] KVM: PPC: mmio: Deliver DSI after emulation failure

[Nicholas Piggin]

Excerpts from Fabiano Rosas's message of January 22, 2022 8:26 am:
> MMIO emulation can fail if the guest uses an instruction that we are
> not prepared to emulate. Since these instructions can be and most
> likely are valid ones, this is (slightly) closer to an access fault
> than to an illegal instruction, so deliver a Data Storage interrupt
> instead of a Program interrupt.
> 
> Suggested-by: Nicholas Piggin <npiggin at gmail.com>
> Signed-off-by: Fabiano Rosas <farosas at linux.ibm.com>
> ---
>  arch/powerpc/kvm/emulate_loadstore.c | 10 +++-------
>  arch/powerpc/kvm/powerpc.c           | 12 ++++++++++++
>  2 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/emulate_loadstore.c b/arch/powerpc/kvm/emulate_loadstore.c
> index 48272a9b9c30..cfc9114b87d0 100644
> --- a/arch/powerpc/kvm/emulate_loadstore.c
> +++ b/arch/powerpc/kvm/emulate_loadstore.c
> @@ -73,7 +73,6 @@ int kvmppc_emulate_loadstore(struct kvm_vcpu *vcpu)
>  {
>  	u32 inst;
>  	enum emulation_result emulated = EMULATE_FAIL;
> -	int advance = 1;
>  	struct instruction_op op;
>  
>  	/* this default type might be overwritten by subcategories */
> @@ -98,6 +97,8 @@ int kvmppc_emulate_loadstore(struct kvm_vcpu *vcpu)
>  		int type = op.type & INSTR_TYPE_MASK;
>  		int size = GETSIZE(op.type);
>  
> +		vcpu->mmio_is_write = OP_IS_STORE(type);
> +
>  		switch (type) {
>  		case LOAD:  {
>  			int instr_byte_swap = op.type & BYTEREV;
> @@ -355,15 +356,10 @@ int kvmppc_emulate_loadstore(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> -	if (emulated == EMULATE_FAIL) {
> -		advance = 0;
> -		kvmppc_core_queue_program(vcpu, 0);
> -	}
> -
>  	trace_kvm_ppc_instr(inst, kvmppc_get_pc(vcpu), emulated);
>  
>  	/* Advance past emulated instruction. */
> -	if (advance)
> +	if (emulated != EMULATE_FAIL)
>  		kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
>  
>  	return emulated;
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 214602c58f13..9befb121dddb 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -305,10 +305,22 @@ int kvmppc_emulate_mmio(struct kvm_vcpu *vcpu)
>  	case EMULATE_FAIL:
>  	{
>  		u32 last_inst;
> +		ulong store_bit = DSISR_ISSTORE;
> +		ulong cause = DSISR_BADACCESS;
>  
> +#ifdef CONFIG_BOOKE
> +		store_bit = ESR_ST;
> +		cause = 0;
> +#endif

BookE can not cause a bad page fault in the guest with ESR bits AFAIKS, 
so it would cause an infinite fault loop here. Maybe stick with the 
program interrupt for BookE with a comment about that here.

And if it could use if (IS_ENABLED()) would be good?

Otherwise looks good, it should do the right thing on BookS.

Thanks,
Nick

>  		kvmppc_get_last_inst(vcpu, INST_GENERIC, &last_inst);
>  		pr_info_ratelimited("KVM: guest access to device memory using unsupported instruction (PID: %d opcode: %#08x)\n",
>  				    current->pid, last_inst);
> +
> +		if (vcpu->mmio_is_write)
> +			cause |= store_bit;
> +
> +		kvmppc_core_queue_data_storage(vcpu, vcpu->arch.vaddr_accessed,
> +					       cause);
>  		r = RESUME_GUEST;
>  		break;
>  	}
> -- 
> 2.34.1
> 
>