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Re: [Qemu-block] [Qemu-devel] Configuring pflash devices for OVMF firmwa

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] Configuring pflash devices for OVMF firmware
Date: Thu, 31 Jan 2019 13:51:50 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Paolo Bonzini <address@hidden> 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!

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