From 9dbd23e42ff0b10c9b02c9e649c76e5228241a8e Mon Sep 17 00:00:00 2001 From: "Borislav Petkov (AMD)" Date: Mon, 14 Aug 2023 21:29:50 +0200 Subject: x86/srso: Explain the untraining sequences a bit more The goal is to eventually have a proper documentation about all this. Signed-off-by: Borislav Petkov (AMD) Link: https://lore.kernel.org/r/20230814164447.GFZNpZ/64H4lENIe94@fat_crate.local --- arch/x86/lib/retpoline.S | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) (limited to 'arch/x86/lib') diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index 5e85da150e96..cd86aeb5fdd3 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -186,6 +186,25 @@ SYM_CODE_START(srso_alias_return_thunk) ud2 SYM_CODE_END(srso_alias_return_thunk) +/* + * Some generic notes on the untraining sequences: + * + * They are interchangeable when it comes to flushing potentially wrong + * RET predictions from the BTB. + * + * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the + * Retbleed sequence because the return sequence done there + * (srso_safe_ret()) is longer and the return sequence must fully nest + * (end before) the untraining sequence. Therefore, the untraining + * sequence must fully overlap the return sequence. + * + * Regarding alignment - the instructions which need to be untrained, + * must all start at a cacheline boundary for Zen1/2 generations. That + * is, instruction sequences starting at srso_safe_ret() and + * the respective instruction sequences at retbleed_return_thunk() + * must start at a cacheline boundary. + */ + /* * Safety details here pertain to the AMD Zen{1,2} microarchitecture: * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for -- cgit v1.2.3