[tip: x86/microcode] x86/microcode: Do not taint when late loading on AMD

[tip-bot2 for Borislav Petkov (AMD)]

The following commit has been merged into the x86/microcode branch of tip:

Commit-ID:     09b951476df9eadf15f2acba7568fa35e4b2313b
Gitweb:        https://git.kernel.org/tip/09b951476df9eadf15f2acba7568fa35e4b2313b
Author:        Borislav Petkov (AMD) <bp@xxxxxxxxx>
AuthorDate:    Fri, 03 Mar 2023 12:46:49 +01:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Sun, 12 Mar 2023 21:32:51 +01:00

x86/microcode: Do not taint when late loading on AMD

Describe why the concurrency issues which late loading poses are not
affecting AMD hardware, after discussing it with hw folks. Thus, do not
taint when late loading on it.

Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20230303114649.18552-1-bp@xxxxxxxxx
---
 Documentation/x86/microcode.rst      | 10 ++++++++++
 arch/x86/kernel/cpu/microcode/core.c |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/Documentation/x86/microcode.rst b/Documentation/x86/microcode.rst
index b627c6f..15b52e2 100644
--- a/Documentation/x86/microcode.rst
+++ b/Documentation/x86/microcode.rst
@@ -208,6 +208,16 @@ Basically there is no way to declare a new microcode update suitable
 for late-loading. This is another one of the problems that caused late
 loading to be not enabled by default.
 
+AMD
+---
+
+Late loading on AMD does not have the concurrency issues described
+above: when loading is attempted on T0, the T1 is quiesced and does not
+execute instructions. Therefore, even if a higher priority interrupt or
+a fault happens, the whole core will see it either before the microcode
+patch has been applied or after. In either case, T0 and T1 will have the
+same microcode revision and nothing intermediate.
+
 Builtin microcode
 =================
 
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 7a329e5..779f705 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -498,7 +498,8 @@ put:
 	if (ret == 0)
 		ret = size;
 
-	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 
 	return ret;
 }