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

[Daniel P . Berrangé]

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
> > boundary.
> > 
> > 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. 

With regards,
Daniel
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