[libvirt] [Qemu-devel] Configuring pflash devices for OVMF firmware

[Markus Armbruster]

Paolo Bonzini <pbonzini at redhat.com> writes:

> On 31/01/19 10:37, Markus Armbruster wrote:
>>>
>>>> Hmm, why is it okay to treat all pfl->cmd values the same when
>>>> secure=on?
>>> But doesn't matter.  You just don't want MMIO mode to be active outside
>>> SMM: all that non-SMM code want to do with the flash is read and execute
>>> it, as far as they're concerned it's just ROM and the command mode is
>>> nonexistent.
>> Out of curiosity: what effect does secure=on have when the device is
>> read-only (pflash_t member ro non-zero)?
>
> Non-SMM code cannot execute commands.  This means two things:
>
> First, in addition to writes, there are nondestructive commands such as
> read device id.  Those are also inaccessible to non-SMM if secure=on.
> Again, for non-SMM code it looks like your old ROM.  This is not
> important but...
>
> ... CFI commands, even commands that are nondestructive or writes that
> fail because of readonly-ness, consist of multiple writes to the flash
> device.  If non-SMM code could issue a partial command, the SMM flash
> driver would likely end up confused.  Therefore it's probably a good
> idea to make all parallel flash devices have secure=on even if the
> content of the flash cannot be damaged, and that's why I never
> considered anything but -global to configure the property.

Makes sense now, thanks!