Re: [PATCH 1/2] kvm: x86: Allow userspace to handle emulation errors

[David Edmondson <dme@dme.org>]

On Tuesday, 2021-04-20 at 07:57:27 -07, Aaron Lewis wrote:

>> >> Why not add a new exit reason, particularly given that the caller has to
>> >> enable the capability to get the relevant data? (It would also remove
>> >> the need for the flag field and any mechanism for packing multiple bits
>> >> of detail into the structure.)
>> >
>> > I considered that, but I opted for the extensibility of the exiting
>> > KVM_EXIT_INTERNAL_ERROR instead.  To me it was six of one or half a
>> > dozen of the other.  With either strategy I still wanted to provide
>> > for future extensibility, and had a flags field in place.  That way we
>> > can add to this in the future if we find something that is missing
>> > (ie: potentially wanting a way to mark dirty pages, possibly passing a
>> > fault address, etc...)
>>
>> How many of the flag based optional fields do you anticipate needing for
>> any one particular exit scenario?
>>
>> If it's one, then using the flags to disambiguate the emulation failure
>> cases after choosing to stuff all of the cases into
>> KVM_EXIT_INTERNAL_ERROR / KVM_INTERNAL_ERROR_EMULATION would be odd.
>>
>> (I'm presuming that it's not one, but don't understand the use case.)
>>
>
> The motivation was to allow for maximum flexibility in the future, and
> not be tied down to something we potentially missed now.  I agree the
> flags aren't needed if we are only adding to what's currently there,
> but they are needed if we want to remove something or pack something
> differently.  I didn't see how I could achieve that without adding a
> flags field.  Seemed like low overhead to be more future proof.

With what you have now, the ndata field seems unnecessary - I should be
able to determine the contents of the rest of the structure based on the
flags. That also suggests to me that using something other than
KVM_INTERNAL_ERROR_EMULATION would make sense.

This comment:

>> >> > + * When using the suberror KVM_INTERNAL_ERROR_EMULATION, these flags are used
>> >> > + * to describe what is contained in the exit struct.  The flags are used to
>> >> > + * describe it's contents, and the contents should be in ascending numerical
>> >> > + * order of the flag values.  For example, if the flag
>> >> > + * KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES is set, the instruction
>> >> > + * length and instruction bytes would be expected to show up first because this
>> >> > + * flag has the lowest numerical value (1) of all the other flags.

originally made me think that the flag-indicated elements were going to
be packed into the remaining space of the structure at a position
depending on which flags are set.

For example, if I add a new flag
KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_CODE, value 2, and then want to
pass back an exit code but *not* instruction bytes, the comment appears
to suggest that the exit code will appear immediately after the flags.

This is contradicted by your other reply:

>> > Just add the fields you need to
>> > the end of emulation_failure struct, increase 'ndata' to the new
>> > count, add a new flag to 'flags' so we know its contents.

Given this, the ordering of flag values does not seem significant - the
structure elements corresponding to a flag value will always be present,
just not filled with relevant data.

dme.
-- 
When you were the brightest star, who were the shadows?