OvmfPkg/README: HTTPS Boot: describe host-side TLS cipher suites forwarding

In QEMU commit range 4abf70a661a5..69699f3055a5 (later fixed up in QEMU
commit 4318432ccd3f), Phil implemented a QEMU facility for exposing the
host-side TLS cipher suite configuration to OVMF. The purpose is to
control the permitted ciphers in the guest's UEFI HTTPS boot. This
complements the forwarding of the host-side crypto policy from the host to
the guest -- the other facet was the set of CA certificates (for which
p11-kit patches had been upstreamed, on the host side).

Mention the new command line options in "OvmfPkg/README".

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Gary Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2852
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200922091827.12617-1-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/README

@@ -303,16 +303,27 @@ and encrypted connection.
 * Besides the trusted certificates, it's also possible to configure the trusted
   cipher suites for HTTPS through another fw_cfg entry: etc/edk2/https/ciphers.
 
-  -fw_cfg name=etc/edk2/https/ciphers,file=<cipher suites>
-
   OVMF expects a binary UINT16 array which comprises the cipher suites HEX
   IDs(*4). If the cipher suite list is given, OVMF will choose the cipher
   suite from the intersection of the given list and the built-in cipher
   suites. Otherwise, OVMF just chooses whatever proper cipher suites from the
   built-in ones.
 
-  While the tool(*5) to create the cipher suite array is still under
-  development, the array can be generated with the following script:
+  - Using QEMU 5.2 or later, QEMU can expose the ordered list of permitted TLS
+    cipher suites from the host side to OVMF:
+
+  -object tls-cipher-suites,id=mysuite0,priority=@SYSTEM \
+  -fw_cfg name=etc/edk2/https/ciphers,gen_id=mysuite0
+
+  (Refer to the QEMU manual and to
+  <https://gnutls.org/manual/html_node/Priority-Strings.html> for more
+  information on the "priority" property.)
+
+  - Using QEMU 5.1 or earlier, the array has to be passed from a file:
+
+  -fw_cfg name=etc/edk2/https/ciphers,file=<cipher suites>
+
+  whose contents can be generated with the following script, for example:
 
   export LC_ALL=C
   openssl ciphers -V \
@@ -337,15 +348,10 @@ and encrypted connection.
      -e 's/^ *0x([0-9A-F]{2}),0x([0-9A-F]{2}) - .*$/\\\\x\1 \\\\x\2/p' \
   | xargs -r -- printf -- '%b' > ciphers.bin
 
-* In the future (after release 2.12), QEMU should populate both above fw_cfg
-  files automatically from the local host configuration, and enable the user
-  to override either with dedicated options or properties.
-
 (*1) See "31.4.1 Signature Database" in UEFI specification 2.7 errata A.
 (*2) p11-kit: https://github.com/p11-glue/p11-kit/
 (*3) efisiglist: https://github.com/rhboot/pesign/blob/master/src/efisiglist.c
 (*4) https://wiki.mozilla.org/Security/Server_Side_TLS#Cipher_names_correspondence_table
-(*5) update-crypto-policies: https://gitlab.com/redhat-crypto/fedora-crypto-policies
 
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