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

[Stefan Hajnoczi]

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?

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