OvmfPkg: add qemu vars documentation

Add documentation for the qemu uefi variable store.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

OvmfPkg/QEMU_PV_VARS.md

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+# QEMU uefi variable store
+
+Starting with version 10.0 (released April 2025) qemu can provide an
+uefi variable store to the guest.
+
+## Motivation
+
+Main difference to the traditional approach to provide pflash storage
+for the variables is that qemu will also handle access control and
+signature verification for authenticated variable updates.  Moving
+that functionality from priviledged guest mode (SMM on x64) to the
+host makes it much easier to support secure boot.  The requirement to
+have some priviledged guest mode for VMs goes away.
+
+## Build the firmware
+
+Support for the qemu uefi variable store is a compile time option.  It
+is disabled by default and can be enabled using the QEMU_PV_VARS
+option.  It makes sense to also enable secure boot support, i.e. build
+with '-D QEMU_PV_VARS=TRUE -D SECURE_BOOT_ENABLE=TRUE'.
+
+Supported platforms:
+
+ * OvmfPkg/OvmfPkgX64.dsc
+ * ArmVirtPkg/ArmVirtQemu.dsc
+ * ArmVirtPkg/ArmVirtQemuKernel.dsc
+ * OvmfPkg/RiscVVirt/RiscVVirtQemu.dsc
+
+## Using the qemu uefi variable store
+
+The variable store is implemented as qemu device and it comes in two
+variants.  The first is `uefi-vars-x64`, for the x64 platform, using
+the 'etc/hardware-info' fw_cfg file for device discovery.  The second
+is `uefi-vars-sysbus`, used by all other platforms, using device tree
+for device discovery.
+
+Simplest way to use it is this:
+
+```
+qemu-system-x86_64 \
+    -bios OVMF.qemuvars.fd \
+    -device uefi-vars-x64 \
+    $otherargs
+```
+
+This runs the uefi variable store without persistence, i.e. the
+variables will be lost on VM poweroff (i.e. qemu exit).  The variables
+will survive reboots though.
+
+QEMU can save the variable store in json format for persistence.  This
+is enabled by passing a filename for the varstore to qemu:
+
+```
+touch /path/to/varstore.json
+qemu-system-x86_64 \
+    -bios OVMF.qemuvars.fd \
+    -device uefi-vars-x64,jsonfile=/path/to/varstore.json \
+    $otherargs
+```
+
+## Enroll secure boot variables
+
+There are two projects which can handle the json format variable
+store: https://gitlab.com/kraxel/virt-firmware and
+https://github.com/awslabs/python-uefivars.
+
+virt-firmware can be installed via `pip install virt-firmware`.  Your
+linux distro might also have packages for you.  On Fedora, RHEL and
+Centos Stream `dnf install virt-firmware` works.
+
+Creating a variable store with secure boot variables enrolled works
+this way:
+
+```
+virt-fw-vars \
+    --secure-boot \
+    --enroll-redhat \
+    --set-dbx /usr/share/edk2/ovmf/DBXUpdate-${date}.x64.bin \
+    --output-json /path/to/varstore.json
+```
+
+This will enroll the standard set of microsoft certificates (both 2011
+and 2023).  There are a number of config options to change default
+behavior, check out `virt-fw-vars --help`.
+
+If your linux distro does not ship the dbx revocation database you can
+get it from https://github.com/microsoft/secureboot_objects/
+(subdirectory `PostSignedObjects/DBX`) instead.