Re: [PATCH 2/2] KVM: x86: On emulation failure, convey the exit reason to userspace

[David Matlack <dmatlack@google.com>]

On Mon, Jun 28, 2021 at 06:31:52PM +0100, David Edmondson wrote:
> To aid in debugging.

Please add more context to the commit message.

> 
> Suggested-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: David Edmondson <david.edmondson@oracle.com>
> ---
>  arch/x86/kvm/x86.c       | 23 +++++++++++++++++------
>  include/uapi/linux/kvm.h |  2 ++
>  2 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8166ad113fb2..48ef0dc68faf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7455,7 +7455,7 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
>  }
>  EXPORT_SYMBOL_GPL(kvm_inject_realmode_interrupt);
>  
> -static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
> +static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu, uint64_t flags)
>  {
>  	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
>  	u32 insn_size = ctxt->fetch.end - ctxt->fetch.data;
> @@ -7466,7 +7466,8 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
>  	run->emulation_failure.ndata = 0;
>  	run->emulation_failure.flags = 0;
>  
> -	if (insn_size) {
> +	if (insn_size &&
> +	    (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES)) {
>  		run->emulation_failure.ndata = 3;
>  		run->emulation_failure.flags |=
>  			KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES;
> @@ -7476,6 +7477,14 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
>  		memcpy(run->emulation_failure.insn_bytes,
>  		       ctxt->fetch.data, insn_size);
>  	}
> +
> +	if (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON) {

This flag is always passed so this check if superfluous. Perhaps change
`int flags` to `bool instruction_bytes` and have it control only whether
the instruction bytes are populated.

> +		run->emulation_failure.ndata = 4;
> +		run->emulation_failure.flags |=
> +			KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON;
> +		run->emulation_failure.exit_reason =
> +			static_call(kvm_x86_get_exit_reason)(vcpu);
> +	}
>  }
>  
>  static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
> @@ -7492,16 +7501,18 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
>  
>  	if (kvm->arch.exit_on_emulation_error ||
>  	    (emulation_type & EMULTYPE_SKIP)) {
> -		prepare_emulation_failure_exit(vcpu);
> +		prepare_emulation_failure_exit(
> +			vcpu,
> +			KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES |
> +			KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);
>  		return 0;
>  	}
>  
>  	kvm_queue_exception(vcpu, UD_VECTOR);
>  
>  	if (!is_guest_mode(vcpu) && static_call(kvm_x86_get_cpl)(vcpu) == 0) {
> -		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> -		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
> -		vcpu->run->internal.ndata = 0;
> +		prepare_emulation_failure_exit(
> +			vcpu, KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);

Should kvm_task_switch and kvm_handle_memory_failure also be updated
like this?

>  		return 0;
>  	}
>  
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 68c9e6d8bbda..3e4126652a67 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -282,6 +282,7 @@ struct kvm_xen_exit {
>  
>  /* Flags that describe what fields in emulation_failure hold valid data. */
>  #define KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES (1ULL << 0)
> +#define KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON       (1ULL << 1)
>  
>  /* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
>  struct kvm_run {
> @@ -404,6 +405,7 @@ struct kvm_run {
>  			__u64 flags;
>  			__u8  insn_size;
>  			__u8  insn_bytes[15];
> +			__u64 exit_reason;

Please document what this field contains, especially since its contents
depend on AMD versus Intel.

>  		} emulation_failure;
>  		/* KVM_EXIT_OSI */
>  		struct {
> -- 
> 2.30.2
>