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

[Eric Blake <eblake@redhat.com>]

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

> 
> 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