On 4.04.25 г. 18:17 ч., Josh Poimboeuf wrote:
On Fri, Apr 04, 2025 at 05:45:37PM +0300, Nikolay Borisov wrote:
On 2.04.25 г. 21:19 ч., Josh Poimboeuf wrote:
IBPB is expected to clear the RSB. However, if X86_BUG_IBPB_NO_RET is
set, that doesn't happen. Make indirect_branch_prediction_barrier()
take that into account by calling __write_ibpb() which already does the
right thing.
I find this changelog somewhat dubious. So zen < 4 basically have
IBPB_NO_RET, your patch 2 in this series makes using SBPB for cores which
have SRSO_NO or if the mitigation is disabled. So if you have a core which
is zen <4 and doesn't use SBPB then what happens?
I'm afraid I don't understand the question. In that case write_ibpb()
uses IBPB and manually clears the RSB.
Actually isn't this patch a noop. The old code simply wrote the value of
x86_pred_cmd to the IA32-PRED_CMD register iff FEATURE_IBPB was set. So
x86_pred_cmd might contain either PRED_CMD_IBPB or PRED_CMD_SBPB,
meaning the correct value was written.
With your change you now call __write_ibpb() which does effectively the
same thing.
