Re: [RFC 2/4] qcow2: add configurations for zoned format extension

[Sam Li <faithilikerun@gmail.com>]

Stefan Hajnoczi <stefanha@redhat.com> 于2023年6月19日周一 22:42写道：
>
> On Mon, Jun 19, 2023 at 06:32:52PM +0800, Sam Li wrote:
> > Stefan Hajnoczi <stefanha@redhat.com> 于2023年6月19日周一 18:10写道：
> > > On Mon, Jun 05, 2023 at 06:41:06PM +0800, Sam Li wrote:
> > > > diff --git a/block/qcow2.h b/block/qcow2.h
> > > > index 4f67eb912a..fe18dc4d97 100644
> > > > --- a/block/qcow2.h
> > > > +++ b/block/qcow2.h
> > > > @@ -235,6 +235,20 @@ typedef struct Qcow2CryptoHeaderExtension {
> > > >      uint64_t length;
> > > >  } QEMU_PACKED Qcow2CryptoHeaderExtension;
> > > >
> > > > +typedef struct Qcow2ZonedHeaderExtension {
> > > > +    /* Zoned device attributes */
> > > > +    BlockZonedProfile zoned_profile;
> > > > +    BlockZoneModel zoned;
> > > > +    uint32_t zone_size;
> > > > +    uint32_t zone_capacity;
> > > > +    uint32_t nr_zones;
> > > > +    uint32_t zone_nr_conv;
> > > > +    uint32_t max_active_zones;
> > > > +    uint32_t max_open_zones;
> > > > +    uint32_t max_append_sectors;
> > > > +    uint8_t padding[3];
> > >
> > > This looks strange. Why is there 3 bytes of padding at the end? Normally
> > > padding would align to an even power-of-two number of bytes like 2, 4,
> > > 8, etc.
> >
> > It is calculated as 3 if sizeof(zoned+zoned_profile) = 8. Else if it's
> > 16, the padding is 2.
>
> I don't understand. Can you explain why there is padding at the end of
> this struct?

The overall size should be aligned with 64 bit, which leaves use one
uint32_t and two fields zoned, zoned_profile. I am not sure the size
of macros here and it used 4 for each. So it makes 3 (*8) + 32 + 8 =
64 in the end. If the macro size is wrong, then the padding will
change as well.

Sam