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

[David Edmondson <david.edmondson@oracle.com>]

On Friday, 2021-07-09 at 21:58:12 GMT, Sean Christopherson wrote:

> On Fri, Jul 02, 2021, David Edmondson wrote:
>> 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:
>> >>  	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?
>
> Doh, the SGX case should have been updated as part of commit c88339d88b0a ("kvm:
> x86: Allow userspace to handle emulation errors").  The easiest fix for SGX would
> be to zero out 'flags', bump ndata, and shift the existing field usage.  That
> would resolve the existing problem of the address being misinterpreted as flags,
> and would play nice _if_ additional flags are added.  I'll send a patch for that.
>
> [...]
>
> Which brings me back to adding another flag when dumping the exit reason.  Unless
> there is a concrete use case for programmatically taking action in reponse to
> failed emulation, e.g. attemping emulation in userspace using insn_bytes+insn_size,
> I think we should not add a flag and instead dump info for debug/triage purposes
> without committing to an ABI.  I.e. define the ABI such that KVM can dump
> arbitrary info in the unused portions of data[].

https://lore.kernel.org/r/20210729133931.1129696-1-david.edmondson@oracle.com
includes both of these suggestions.