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

From: Paolo Bonzini
Subject: Re: [Qemu-block] [Qemu-devel] Configuring pflash devices for OVMF firmware
Date: Thu, 31 Jan 2019 13:10:33 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

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.


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