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

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Message-ID: <20251125002130.2dfsa7buv4aps5js@desk>
Date: Mon, 24 Nov 2025 16:21:30 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: x86@...nel.org, David Kaplan <david.kaplan@....com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Asit Mallick <asit.k.mallick@...el.com>,
	Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 02/11] x86/bhi: Move the BHB sequence to a macro for
 reuse

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@...ux.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.