Re: [PATCH v8 01/10] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()

[Borislav Petkov]

On Tue, Mar 24, 2026 at 11:16:36AM -0700, 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
> 	       can be used to mitigate BHI. It can also be used to mitigate
> 	       BHI variant of VMSCAPE. This is not yet implemented in
> 	       Linux.
> 
>   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.
> 
> On Alder Lake and newer CPUs, eIBRS isolates the indirect targets between
> guest and host. But when affected by the BHI variant of VMSCAPE, a guest's
> branch history may still influence indirect branches in userspace. This
> also means the big hammer IBPB could be replaced with a cheaper option that
> clears the BHB at exit-to-userspace after a VMexit.
> 
> In preparation for adding the support for BHB sequence (without LFENCE) on
> newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
> executed. Allow callers to decide whether they need the LFENCE or
> not. This adds a few extra bytes to the call sites, but it obviates
> the need for multiple variants of clear_bhb_loop().

Claude, please add proper articles where they're missing in the above text:

"Currently, the BHB clearing sequence is followed by an LFENCE to prevent
transient execution of subsequent indirect branches prematurely. However, the
LFENCE barrier could be unnecessary in certain cases. For example, when the
kernel is using the BHI_DIS_S mitigation, and BHB clearing is only needed for
userspace. In such cases, the 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, a longer version of the BHB clearing sequence
    can be used to mitigate BHI. It can also be used to mitigate the BHI
    variant of VMSCAPE. This is not yet implemented in Linux.

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.

On Alder Lake and newer CPUs, eIBRS isolates the indirect targets between
guest and host. But when affected by the BHI variant of VMSCAPE, a guest's
branch history may still influence indirect branches in userspace. This also
means the big hammer IBPB could be replaced with a cheaper option that clears
the BHB at exit-to-userspace after a VMexit.

In preparation for adding the support for the BHB sequence (without LFENCE) on
newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
executed. Allow callers to decide whether they need the LFENCE or not. This
adds a few extra bytes to the call sites, but it obviates the need for
multiple variants of clear_bhb_loop()."

Reads proper to me. Use it for your next revision pls.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette