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

[David Edmondson]

On Wednesday, 2021-06-30 at 16:48:42 UTC, David Matlack wrote:

> 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.

Okay.

>> 
>> Suggested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
>> Signed-off-by: David Edmondson <david.edmondson@xxxxxxxxxx>
>> ---
>>  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.

Okay.

>> +		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?

Will do in v2.

sgx_handle_emulation_failure() seems like an existing user of
KVM_INTERNAL_ERROR_EMULATION that doesn't follow the new protocol (use
the emulation_failure part of the union).

Sean: If I add another flag for this case, what is the existing
user-level consumer?

>>  		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.

Okay.

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

dme.
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