Re: [RFC PATCH 3/6] hw/sd/sdcard: Do not use legal address '0' for INVALID_ADDRESS

[Markus Armbruster]

Philippe Mathieu-Daudé <philmd@redhat.com> writes:

> +Paolo & Kevin.
>
> On 9/21/20 10:40 AM, Markus Armbruster wrote:
>> Philippe Mathieu-Daudé <f4bug@amsat.org> writes:
>> 
>>> As it is legal to WRITE/ERASE the address/block 0,
>>> change the value of this definition to an illegal
>>> address: UINT32_MAX.
>>>
>>> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>> ---
>>> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> Cc: Markus Armbruster <armbru@redhat.com>
>>>
>>> Same problem I had with the pflash device last year...
>>> This break migration :(
>>> What is the best way to do this?
>> 
>> Remind me: did we solve the problem with pflash, and if yes, how?
>
> No we can't. The best I could do is add a comment and as this
> is not fixable. See commit aba53a12bd5: ("hw/block/pflash_cfi01:
> Document use of non-CFI compliant command '0x00'").
>
> I now consider the device in maintenance-only
> mode and won't add any new features.
>
> I started working on a new implementation, hoping it can be a
> drop in replacement. Laszlo still has hope that QEMU pflash
> device will support sector locking so firmware developers could
> test upgrading fw in VMs.
>
> Back to the SDcard, it might be less critical, so a migration
> breaking change might be acceptable. I'm only aware of Paolo
> and Kevin using this device for testing. Not sure of its
> importance in production.

Neither am I.

Which machine types include this device by default?

How can a non-default device be added, and to which machine types?

I gather the fix changes device state incompatibly.  Always, or only in
certain states?  I'm asking because if device state remains compatible
most of the time, we might be able use subsection trickery to keep
migration working most of the time.  Has been done before, I think.