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 originally made me think that the flag-indicated elements were going to be packed into the remaining space of the structure at a position depending on which flags are set.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