Re: [Qemu-devel] [qemu PATCH] hw/i386/pc_sysfw: support more than one flash drive

[Markus Armbruster <armbru@redhat.com>]

Laszlo Ersek <lersek@redhat.com> writes:

> On 11/27/13 14:52, Markus Armbruster wrote:
>> Jordan Justen <jljusten@gmail.com> writes:
>> 
>>> On Tue, Nov 26, 2013 at 5:32 AM, Laszlo Ersek <lersek@redhat.com> wrote:
>>>> On 11/26/13 13:36, Markus Armbruster wrote:
>>>>
>>>>> Your stated purpose for multiple -pflash:
>>>>>
>>>>>     This accommodates the following use case: suppose that OVMF is split in
>>>>>     two parts, a writeable host file for non-volatile variable
>>>>> storage, and a
>>>>>     read-only part for bootstrap and decompressible executable code.
>>>>>
>>>>> Such a split between writable part and read-only part makes sense to me.
>>>>> How is it done in physical hardware?  Single device with configurable
>>>>> write-protect, or two separate devices?
>>>>
>>>> (Jordan could help more.)
>>>>
>>>> Likely one device that's fully writeable.
>>>
>>> Most parts will have a dedicated read-only line.
>>>
>>> Many devices have 'block-locking' that will make some subset of blocks
>>> read-only until a reset.
>>>
>>> In addition to this, many chipsets will allow flash writes to be
>>> protected by triggering SMM when a flash write occurs.
>>>
>>> Using multiple chips are less common due to cost, but this is not a
>>> factor for QEMU. :)
>> 
>> Should we stick to what real hardware does?  Single device, perhaps with
>> block locking.
>
> I can't back a single flash device with two drives (= two host-side
> files), which is the incentive for this change.

There's no fundamental reason why a single device model instance could
not be backed by two block backends, named by two drive properties.

I'm not claiming this is the best solution, just offering it for
consideration.