On Thu, Jan 09, 2025 at 03:52:48PM -0300, Fabiano Rosas wrote:
> Currently, if an array of pointers contains a NULL pointer, that
> pointer will be encoded as '0' in the stream. Since the JSON writer
> doesn't define a "pointer" type, that '0' will now be an uint8, which
> is different from the original type being pointed to, e.g. struct.
>
> (we're further calling uint8 "nullptr", but that's irrelevant to the
> issue)
>
> That mixed-type array shouldn't be compressed, otherwise data is lost
> as the code currently makes the whole array have the type of the first
> element:
>
> css = {NULL, NULL, ..., 0x5555568a7940, NULL};
>
> {"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
> "version": 1, "fields": [
> ...,
> {"name": "css", "array_len": 256, "type": "nullptr", "size": 1},
> ...,
> ]}
>
> In the above, the valid pointer at position 254 got lost among the
> compressed array of nullptr.
>
> While we could disable the array compression when a NULL pointer is
> found, the JSON part of the stream still makes part of downtime, so we
> should avoid writing unecessary bytes to it.
>
> Keep the array compression in place, but if NULL and non-NULL pointers
> are mixed break the array into several type-contiguous pieces :
>
> css = {NULL, NULL, ..., 0x5555568a7940, NULL};
>
> {"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
> "version": 1, "fields": [
> ...,
> {"name": "css", "array_len": 254, "type": "nullptr", "size": 1},
> {"name": "css", "type": "struct", "struct": {"vmsd_name":
> "s390_css_img", ... }, "size": 768},
> {"name": "css", "type": "nullptr", "size": 1},
> ...,
> ]}
>
> Now each type-discontiguous region will become a new JSON entry. The
> reader should interpret this as a concatenation of values, all part of
> the same field.
>
> Parsing the JSON with analyze-script.py now shows the proper data
> being pointed to at the places where the pointer is valid and
> "nullptr" where there's NULL:
>
> "s390_css (14)": {
> ...
> "css": [
> "nullptr",
> "nullptr",
> ...
> "nullptr",
> {
> "chpids": [
> {
> "in_use": "0x00",
> "type": "0x00",
> "is_virtual": "0x00"
> },
> ...
> ]
> },
> "nullptr",
> }
>
> Signed-off-by: Fabiano Rosas <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
--
Peter Xu
