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

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

On Thu, Feb 13, 2020 at 06:47:10AM -0600, Eric Blake wrote:
> On 2/13/20 2:01 AM, Roman Kagan wrote:
> > 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.
> 
> You've got bigger problems than what is visible to the guest. block/qcow2.c
> operates on a cluster at a time; if you are stating that it now requires
> reading multiple clusters to operate on one, qcow2 will have to do lots of
> wasteful read-modify-write cycles.

I'm failing to see how this is supposed to happen.  The guest will issue
requests bigger than the cluster size; why would it cause RMW?

Big logical_block_size would cause RMW in the guest if it wants to
perform smaller writes, but that's up to the user to take this tradeoff,
isn't it?

> You really need a strong reason to
> support a maximum larger than 2M other than just "so the guest can
> experiment with it".

Do I get you right that your suggestion is to cap the block size
property at 2MB?

Thanks,
Roman.

> > 
> > 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.
> > 
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
>