[libvirt PATCH 1/5] docs: explain that some UEFI images can use 'rom' instead of 'pflash'

[Daniel P. Berrangé]

The normal requirements for UEFI firmware images are to support
persistence of variables, either in the main image, or more typically in
a separate NVRAM file.

In a confidential computing environment, however, persistence of
variables can cause trust issues and prevent measurement of the firmware
during boot up. For these scenarios some UEFI images will disable
persistence of variables. To use such images the loader type must be set
to 'rom' instead of 'pflash'.

Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
---
 docs/formatdomain.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index c0b2d935f3..cd818c1ded 100644
--- a/docs/formatdomain.rst
+++ b/docs/formatdomain.rst
@@ -214,10 +214,14 @@ harddisk, cdrom, network) determining where to obtain/find the boot image.
    the fact that the image should be writable or read-only. The second attribute
    ``type`` accepts values ``rom`` and ``pflash``. It tells the hypervisor where
    in the guest memory the file should be mapped. For instance, if the loader
-   path points to an UEFI image, ``type`` should be ``pflash``. Moreover, some
-   firmwares may implement the Secure boot feature. Attribute ``secure`` can be
-   used to tell the hypervisor that the firmware is capable of Secure Boot feature.
-   It cannot be used to enable or disable the feature itself in the firmware.
+   path points to an UEFI image, ``type`` would normally be ``pflash`` to
+   enable support for persistence of firmware variables. Moreover, some
+   firmwares may implement the Secure boot feature. Some UEFI images intended
+   for use with confidential computing environments like AMD SEV will disable
+   persistence of variables, and would thus require ``type`` to be ``rom``.
+   Attribute ``secure`` can be used to tell the hypervisor that the firmware
+   is capable of Secure Boot feature. It cannot be used to enable or disable
+   the feature itself in the firmware.
    :since:`Since 2.1.0`
 ``nvram``
    Some UEFI firmwares may want to use a non-volatile memory to store some
-- 
2.33.1