[RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for AMD

[Amit Shah <amit@kernel.org>]

From: Amit Shah <amit.shah@amd.com>

AMD CPUs do not fall back to the BTB when the RSB underflows for RET
address speculation.  AMD CPUs have not needed to stuff the RSB for
underflow conditions.

The RSB poisoning case is addressed by RSB filling - clean up the FIXME
comment about it.

Signed-off-by: Amit Shah <amit.shah@amd.com>
---
 arch/x86/kernel/cpu/bugs.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47a01d4028f6..0aa629b5537d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1828,9 +1828,6 @@ static void __init spectre_v2_select_mitigation(void)
 	 *    speculated return targets may come from the branch predictor,
 	 *    which could have a user-poisoned BTB or BHB entry.
 	 *
-	 *    AMD has it even worse: *all* returns are speculated from the BTB,
-	 *    regardless of the state of the RSB.
-	 *
 	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack
 	 *    scenario is mitigated by the IBRS branch prediction isolation
 	 *    properties, so the RSB buffer filling wouldn't be necessary to
@@ -1852,8 +1849,6 @@ static void __init spectre_v2_select_mitigation(void)
 	 *
 	 * So to mitigate all cases, unconditionally fill RSB on context
 	 * switches.
-	 *
-	 * FIXME: Is this pointless for retbleed-affected AMD?
 	 */
 	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
-- 
2.47.0