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Re: [PATCH V2] hw/riscv: virt: Remove size restriction for pflash

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH V2] hw/riscv: virt: Remove size restriction for pflash
Date: Tue, 8 Nov 2022 15:12:42 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.4.1

On 7/11/22 18:34, Daniel P. Berrangé wrote:
On Mon, Nov 07, 2022 at 06:32:01PM +0100, Andrew Jones wrote:
On Mon, Nov 07, 2022 at 04:19:10PM +0000, Daniel P. Berrangé wrote:
On Mon, Nov 07, 2022 at 03:50:44PM +0000, Alex Bennée wrote:

Sunil V L <sunilvl@ventanamicro.com> writes:

On Mon, Nov 07, 2022 at 01:06:38PM +0000, Peter Maydell wrote:
On Mon, 7 Nov 2022 at 13:03, Sunil V L <sunilvl@ventanamicro.com> wrote:

The pflash implementation currently assumes fixed size of the
backend storage. Due to this, the backend storage file needs to be
exactly of size 32M. Otherwise, there will be an error like below.

"device requires 33554432 bytes, block backend provides 4194304 bytes"

Fix this issue by using the actual size of the backing store.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>

Do you really want the flash device size presented to the guest
to be variable depending on what the user passed as a block backend?
I don't think this is how we handle flash devices on other boards...

Hi Peter,

x86 appears to support variable flash but arm doesn't. What is
the reason for not supporting variable size flash in arm?

If I recall from the last time we went around this is was the question
of what you should pad it with.

Padding is a very good thing from the POV of upgrades. Firmware has shown
a tendancy to change (grow) over time, and the size has an impact of the
guest ABI/live migration state.

To be able to live migrate, or save/restore to/from files, then the machine
firmware size needs to be sufficient to cope with future size changes of
the firmware. The best way to deal with this is to not use the firmware
binaries' minimum compiled size, but instead to pad it upto a higher

Enforcing such padding is a decent way to prevent users from inadvertantly
painting themselves into a corner with a very specific firmware binary
size at initial boot.

Padding is a good idea, but too much causes other problems. When building
lightweight VMs which may pull the firmware image from a network,
AArch64 VMs require 64MB of mostly zeros to be transferred first, which
can become a substantial amount of the overall boot time[*]. Being able to
create images smaller than the total flash device size, but still add some
pad for later growth, seems like the happy-medium to shoot for.

QEMU configures the firmware using -blockdev, so can use any file
format that QEMU supports at the block layer.  IOW, you can store
the firmware in a qcow2 file and thus you will never fetch any
of the padding zeros to be transferred.  That said I'm not sure
that libvirt supports anything other than a raw file today.

Drew might be referring to:


 > Currently ARM UEFI images are typically built as 2MB/768kB flash
 > images for code and variables respectively. These images are both
 > then padded out to 64MB before being loaded by QEMU.
 > Because the images are 64MB each, QEMU allocates 128MB of memory to
 > read them, and then proceeds to read all 128MB from disk (dirtying
 > the memory). Of this 128MB less than 3MB is useful - the rest is
 > zero padding.
 > On a machine with 100 VMs this wastes over 12GB of memory.

See previous attempts:
- Huawei
- Tencent
- Oracle
- Red Hat

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