linux-kernel - Re: [PATCH v4 01/11] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb

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Message-ID: <0541b782-5289-426a-ad76-83ad4b3a3d6c@suse.com>
Date: Thu, 20 Nov 2025 18:58:52 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....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>, 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 01/11] x86/bhi: x86/vmscape: Move LFENCE out of
 clear_bhb_loop()

On 11/20/25 18:56, Pawan Gupta wrote:
> On Thu, Nov 20, 2025 at 06:15:32PM +0200, Nikolay Borisov wrote:
>>
>>
>> On 11/20/25 08:17, Pawan Gupta wrote:
>>> Currently, BHB clearing sequence is followed by an LFENCE to prevent
>>> transient execution of subsequent indirect branches prematurely. However,
>>> LFENCE barrier could be unnecessary in certain cases. For example, when
>>> kernel is using BHI_DIS_S mitigation, and BHB clearing is only needed for
>>> userspace. In such cases, LFENCE is redundant because ring transitions
>>> would provide the necessary serialization.
>>>
>>> Below is a quick recap of BHI mitigation options:
>>>
>>>     On Alder Lake and newer
>>>
>>>     - BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
>>>                  performance overhead.
>>>     - Long loop: Alternatively, longer version of BHB clearing sequence
>>> 	       on older processors can be used to mitigate BHI. This
>>> 	       is not yet implemented in Linux.
>>
>> I find this description of the Long loop on "ALder lake and newer" somewhat
>> confusing, as you are also referring "older processors". Shouldn't the
>> longer sequence bet moved under "On older CPUs" heading? Or perhaps it must
>> be expanded to say that the long sequence could work on Alder Lake and newer
>> CPUs as well as on older cpus?
> 
> Ya, it needs to be rephrased. Would dropping "on older processors" help?
> 
>      - Long loop: Alternatively, longer version of BHB clearing sequence
> 		 can be used to mitigate BHI. This is not yet implemented
> 		 in Linux.
> 

nit: Perhaps a sentence about why long loop version might be used on 
newer parts in certain cases or why it shouldn't.

>>>
>>>     On older CPUs
>>>
>>>     - Short loop: Clears BHB at kernel entry and VMexit.
> 
> And also talk about "Long loop" effectiveness here:
> 
>      On older CPUs
> 
>      - Short loop: Clears BHB at kernel entry and VMexit. The "Long loop"
> 		  is effective on older CPUs as well, but should be avoided
> 		  because of unnecessary overhead.
>> <snip>

In any case it's much better and indeed clear!

Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>