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

From: Eric Blake
Subject: Re: [PATCH] block: make BlockConf.*_size properties 32-bit
Date: Wed, 12 Feb 2020 15:44:19 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

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,...

Make them 32 bit instead and lift the limitation.

Signed-off-by: Roman Kagan <address@hidden>
  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, 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.

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

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