Re: [PATCH] block: make BlockConf.*_size properties 32-bit

[Roman Kagan <rvkagan@yandex-team.ru>]

On Wed, Feb 12, 2020 at 03:44:19PM -0600, Eric Blake wrote:
> On 2/11/20 5:54 AM, Roman Kagan wrote:
> > Devices (virtio-blk, scsi, etc.) and the block layer are happy to use
> > 32-bit for logical_block_size, physical_block_size, and min_io_size.
> > However, the properties in BlockConf are defined as uint16_t limiting
> > the values to 32768.
> > 
> > This appears unnecessary tight, and we've seen bigger block sizes handy
> > at times.
> 
> What larger sizes?  I could see 64k or maybe even 1M block sizes,...

We played exactly with these two :)

> > 
> > Make them 32 bit instead and lift the limitation.
> > 
> > Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
> > ---
> >   hw/core/qdev-properties.c    | 21 ++++++++++++---------
> >   include/hw/block/block.h     |  8 ++++----
> >   include/hw/qdev-properties.h |  2 +-
> >   3 files changed, 17 insertions(+), 14 deletions(-)
> > 
> > diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
> > index 7f93bfeb88..5f84e4a3b8 100644
> > --- a/hw/core/qdev-properties.c
> > +++ b/hw/core/qdev-properties.c
> > @@ -716,30 +716,32 @@ const PropertyInfo qdev_prop_pci_devfn = {
> >   /* --- blocksize --- */
> > +#define MIN_BLOCK_SIZE 512
> > +#define MAX_BLOCK_SIZE 2147483648
> 
> ...but 2G block sizes are going to have tremendous performance problems.
> 
> I'm not necessarily opposed to the widening to a 32-bit type, but think you
> need more justification or a smaller number for the max block size,

I thought any smaller value would just be arbitrary and hard to reason
about, so I went ahead with the max value that fit in the type and could
be made visibile to the guest.

Besides this is a property that is set explicitly, so I don't see a
problem leaving this up to the user.

> particularly since qcow2 refuses to use cluster sizes larger than 2M and it
> makes no sense to allow a block size larger than a cluster size.

This still doesn't contradict passing a bigger value to the guest, for
experimenting if nothing else.

Thanks,
Roman.