On Tue, 5 Nov 2024 at 18:30, Peter Xu <pet...@redhat.com> wrote:
> https://www.qemu.org/docs/master/devel/qapi-code-gen.html
>
> Sometimes, the behaviour of QEMU changes compatibly, but without a
> change in the QMP syntax (usually by allowing values or operations
> that previously resulted in an error). QMP clients may still need
> to know whether the extension is available.
>
> For this purpose, a list of features can be specified for
> definitions, enumeration values, and struct members. Each feature
> list member can either be { 'name': STRING, '*if': COND }, or
> STRING, which is shorthand for { 'name': STRING }.
* I see, okay.
> It's a legacy issue as not all features are developed together, and that
> was planned to be fixed together with handshake. I think the handshake
> could introduce one header on top to pair channels.
>
> IMHO it is an overkill to add a feature now if it works even if tricky,
> because it's not the 1st day it was tricky. And we're going to have another
> header very soon..
* See, current (this series) 'if' conditional in the
migration_ioc_process_incoming() function is simple as:
if (qio_channel_has_feature(ioc,
QIO_CHANNEL_FEATURE_READ_MSG_PEEK)) { peek_magic_bytes() ... }
If we don't send magic value for the postcopy channel, then we avoid
peeking into magic bytes when postcopy is enabled, because otherwise
thread will block peeking into the magic bytes, so the 'if'
conditional becomes:
if (migrate_multifd() && !migrate_postcopy() &&
qio_channel_has_feature (...) ) {
peek_magic_bytes()
...
} else {
When migrate_postcopy() is true
It'll reach here not only for the 'Postcopy' channel, but even
for the 'default' and 'multifd' channels which send the magic bytes.
Then here again we'll need to identify different channels, right?
}
* Let's not make it so complex. Let's send the magic value for the
postcopy channel and simplify it. If 'handshake' feature is going to
redo it, so be it, what's the difference? OR maybe we can align it
with the 'handshake' feature or as part of it or something like that.
@Fabiano Rosas : may I know more about the 'handshake' feature? What
it'll do and not do?
Thank you.
---
- Prasad
