Re: [PATCH v4 02/11] x86/bhi: Move the BHB sequence to a macro for reuse

Pawan Gupta
2025-11-25T00:21:35Z

On Wed, Nov 19, 2025 at 10:18:04PM -0800, Pawan Gupta wrote:
> In preparation to make clear_bhb_loop() work for CPUs with larger BHB, move
> the sequence to a macro. This will allow setting the depth of BHB-clearing
> easily via arguments.
>
> No functional change intended.
>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
> arch/x86/entry/entry_64.S | 37 +++++++++++++++++++++++--------------
> 1 file changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 886f86790b4467347031bc27d3d761d5cc286da1..a62dbc89c5e75b955ebf6d84f20d157d4bce0253 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1499,11 +1499,6 @@ SYM_CODE_END(rewind_stack_and_make_dead)
> * from the branch history tracker in the Branch Predictor, therefore removing
> * user influence on subsequent BTB lookups.
> *
> - * It should be used on parts prior to Alder Lake. Newer parts should use the
> - * BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
> - * virtualized on newer hardware the VMM should protect against BHI attacks by
> - * setting BHI_DIS_S for the guests.
> - *
> * CALLs/RETs are necessary to prevent Loop Stream Detector(LSD) from engaging
> * and not clearing the branch history. The call tree looks like:
> *
> @@ -1532,10 +1527,7 @@ SYM_CODE_END(rewind_stack_and_make_dead)
> * Note, callers should use a speculation barrier like LFENCE immediately after
> * a call to this function to ensure BHB is cleared before indirect branches.
> */
> -SYM_FUNC_START(clear_bhb_loop)
> - ANNOTATE_NOENDBR
> - push %rbp
> - mov %rsp, %rbp
> +.macro CLEAR_BHB_LOOP_SEQ
> movl $5, %ecx
> ANNOTATE_INTRA_FUNCTION_CALL
> call 1f
> @@ -1545,15 +1537,16 @@ SYM_FUNC_START(clear_bhb_loop)
> * Shift instructions so that the RET is in the upper half of the
> * cacheline and don't take the slowpath to its_return_thunk.
> */
> - .skip 32 - (.Lret1 - 1f), 0xcc
> + .skip 32 - (.Lret1_\@ - 1f), 0xcc
> ANNOTATE_INTRA_FUNCTION_CALL
> 1: call 2f
> -.Lret1: RET
> +.Lret1_\@:
> + RET
> .align 64, 0xcc
> /*
> - * As above shift instructions for RET at .Lret2 as well.
> + * As above shift instructions for RET at .Lret2_\@ as well.
> *
> - * This should be ideally be: .skip 32 - (.Lret2 - 2f), 0xcc
> + * This should ideally be: .skip 32 - (.Lret2_\@ - 2f), 0xcc
> * but some Clang versions (e.g. 18) don't like this.
> */
> .skip 32 - 18, 0xcc
> @@ -1564,8 +1557,24 @@ SYM_FUNC_START(clear_bhb_loop)
> jnz 3b
> sub $1, %ecx
> jnz 1b
> -.Lret2: RET
> +.Lret2_\@:
> + RET
> 5:
> +.endm
> +
> +/*
> + * This should be used on parts prior to Alder Lake. Newer parts should use the
> + * BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
> + * virtualized on newer hardware the VMM should protect against BHI attacks by
> + * setting BHI_DIS_S for the guests.
> + */
> +SYM_FUNC_START(clear_bhb_loop)
> + ANNOTATE_NOENDBR
> + push %rbp
> + mov %rsp, %rbp
> +
> + CLEAR_BHB_LOOP_SEQ
> +
> pop %rbp
> RET
> SYM_FUNC_END(clear_bhb_loop)

Dropping this and the next patch, they are not needed with globals for BHB
loop count.