Peter Xu <pet...@redhat.com> writes:
> On Tue, Dec 05, 2023 at 09:44:12AM -0300, Fabiano Rosas wrote:
>> Peter Xu <pet...@redhat.com> writes:
>>
>> > On Mon, Dec 04, 2023 at 06:09:16PM -0300, Fabiano Rosas wrote:
>> >> Right, I got your point. I just think we could avoid designing this new
>> >> string format by creating new fields with the extra space:
>> >>
>> >> typedef struct QEMU_PACKED {
>> >> uint32_t size;
>> >> uint8_t runstate[50];
>> >> uint8_t unused[50];
>> >> RunState state;
>> >> bool received;
>> >> } GlobalState;
>> >>
>> >> In my mind this works seamlessly, or am I mistaken?
>> >
>> > I think what you proposed should indeed work.
>> >
>> > Currently it's:
>> >
>> > .fields = (VMStateField[]) {
>> > VMSTATE_UINT32(size, GlobalState),
>> > VMSTATE_BUFFER(runstate, GlobalState),
>> > VMSTATE_END_OF_LIST()
>> > },
>> >
>> > I had a quick look at vmstate_info_buffer, it mostly only get()/put() those
>> > buffers with its sizeof(), so looks all fine. For sure in all cases we'd
>> > better test it to verify.
>> >
>> > One side note is since we so far use qapi_enum_parse() for the runstate, I
>> > think the "size" is not ever used..
>> >
>> > If we do want a split, IMHO we can consider making runstate[] even smaller
>> > to just free up the rest spaces all in one shot:
>> >
>> > typedef struct QEMU_PACKED {
>
> [1]
>
>> > uint32_t size;
>> > /*
>> > * Assuming 16 is good enough to fit all possible runstate strings..
>> > * This field must be a string ending with '\0'.
>> > */
>> > uint8_t runstate[16];
>> > /* 0x00 when QEMU doesn't support it, or "0"/"1" to reflect its
>> > state */
>> > uint8_t vm_was_suspended[1];
>> > /*
>> > * Still free of use space. Note that we only have 99 bytes for use
>> > * because the last byte (the 100th byte) must be zero due to legacy
>> > * reasons, if not it may be set to zero after loaded on dest QEMU.
>> > */
>>
>> I'd add a 'uint8_t reserved;' to go along with this comment instead of
>> leaving a hole.
>
> Note that "struct GlobalState" is not a binary format but only some
> internal storage, what really matters is vmstate_globalstate. Here the
> "uint8_reserved" will be a pure waste of 1 byte in QEMU binary, imho.
>
I prefer wasting the byte and make the code more obvious to people who
might not immediately understand what's going on. We could even
assert(!global_state.reserved) to sanity check the assumption. Anyway,
that's minor, I'm fine with it either way.
> I think I just copied what you had previously and extended it, logically I
> don't think we ever need QEMU_PACKED right above [1]. We can also drop
> "size" directly here, but this can be done later.
Ah right, I was initially thinking of letting the new qemu overrun
runstate[16] so we wouldn't have to change the code. But that's indeed
not necessary, your additions to the vmstate make it ok. Thanks.
