Commit 9dbd23e4 authored by Borislav Petkov (AMD)'s avatar Borislav Petkov (AMD)
Browse files

x86/srso: Explain the untraining sequences a bit more 



The goal is to eventually have a proper documentation about all this.

Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814164447.GFZNpZ/64H4lENIe94@fat_crate.local
parent 864bcaa3

arch/x86/lib/retpoline.S

+19 −0
Original line number Diff line number Diff line
@@ -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