linux-kernel - Re: [PATCH v6 2/9] x86/bhi: Make clear_bhb

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Message-ID: <fdb0772c-96b8-4772-926d-0d25f7168554@suse.com>
Date: Wed, 10 Dec 2025 14:31:31 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 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 v6 2/9] x86/bhi: Make clear_bhb_loop() effective on newer
 CPUs

On 2.12.25 г. 8:19 ч., Pawan Gupta wrote:
> As a mitigation for BHI, clear_bhb_loop() executes branches that overwrites
> the Branch History Buffer (BHB). On Alder Lake and newer parts this
> sequence is not sufficient because it doesn't clear enough entries. This
> was not an issue because these CPUs have a hardware control (BHI_DIS_S)
> that mitigates BHI in kernel.
> 
> BHI variant of VMSCAPE requires isolating branch history between guests and
> userspace. Note that there is no equivalent hardware control for userspace.
> To effectively isolate branch history on newer CPUs, clear_bhb_loop()
> should execute sufficient number of branches to clear a larger BHB.
> 
> Dynamically set the loop count of clear_bhb_loop() such that it is
> effective on newer CPUs too. Use the hardware control enumeration
> X86_FEATURE_BHI_CTRL to select the appropriate loop count.
> 
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>

nit: My RB tag is incorrect, while I did agree with Dave's suggestion to 
have global variables for the loop counts I haven't' really seen the 
code so I couldn't have given my RB on something which I haven't seen 
but did agree with in principle.

Now that I have seen the code I'm willing to give my :

Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
> ---
>   arch/x86/entry/entry_64.S | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 886f86790b4467347031bc27d3d761d5cc286da1..9f6f4a7c5baf1fe4e3ab18b11e25e2fbcc77489d 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1536,7 +1536,11 @@ SYM_FUNC_START(clear_bhb_loop)
>   	ANNOTATE_NOENDBR
>   	push	%rbp
>   	mov	%rsp, %rbp
> -	movl	$5, %ecx
> +
> +	/* loop count differs based on BHI_CTRL, see Intel's BHI guidance */
> +	ALTERNATIVE "movl $5,  %ecx; movl $5, %edx",	\
> +		    "movl $12, %ecx; movl $7, %edx", X86_FEATURE_BHI_CTRL

nit: Just

> +
>   	ANNOTATE_INTRA_FUNCTION_CALL
>   	call	1f
>   	jmp	5f
> @@ -1557,7 +1561,7 @@ SYM_FUNC_START(clear_bhb_loop)
>   	 * but some Clang versions (e.g. 18) don't like this.
>   	 */
>   	.skip 32 - 18, 0xcc
> -2:	movl	$5, %eax
> +2:	movl	%edx, %eax
>   3:	jmp	4f
>   	nop
>   4:	sub	$1, %eax
>