Re: [PATCH] add tlbi=off to disable INVLPGB

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 29, 2026 at 06:44:12PM -0700, Borislav Petkov wrote:
> On Wed, Jul 29, 2026 at 08:43:41PM -0400, Rik van Riel wrote:
> > With the recently found INVLPGB / TLBSYNC issue, there has been some
> > interest in disabling invlpgb TLB flushing, in order to rule out
> > that CPU issue as a cause of userspace crashes.
> > 
> > The top search result shows "clearcpuid=invlpgb"
> > 
> > However, invlpgb is not actually in x86_cap_flags, so booting with
> > clearcpuid=invlpgb results in an error message:
> > 
> >   clearcpuid: unknown CPU flag: invlpgb
> > 
> > Add a kernel commandline option to turn off AMD TLBI, to make it
> > easier to rule out INVLPGB as a cause of userspace crashes.
> > 
> > Verified by booting a Bergamo system with tlbi=off and checking
> > that the INVLPGB bit is clear in boot_cpu_data.x86_capability
> > 
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> > Suggested-by: Borislav Petkov <bp@alien8.de>
> > ---
> >  arch/x86/mm/init_64.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> 
> I'll take that and raise you with v2:
> 
> The rationale for moving it to amd.c is that it won't be there on builds which
> don't enable AMD code. And we could still raise it up somewhere
> vendor-agnostic later if the other vendor wants to be able to disable their
> version if TLB invalidation glue implementation.
> 
> From: Rik van Riel <riel@surriel.com>
> Date: Wed, 29 Jul 2026 20:43:41 -0400
> Subject: [PATCH] x86/CPU/AMD: Add a tlbi= cmdline switch
> 
> With the recently found INVLPGB / TLBSYNC issue, there has been some
> interest in disabling INVLPGB-based TLB flushing, in order to rule out
> that CPU issue as a cause of userspace crashes.
> 
> Add a kernel command line option to control AMD TLBI.
> 
>   [ bp: Rewrite commit message, move to amd.c, add documentation. ]
> 
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
> Link: https://patch.msgid.link/20260729204341.3eb0b5ea@fangorn
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  5 +++++
>  arch/x86/kernel/cpu/amd.c                       | 10 ++++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 87bb1fb31696..4e3c5fa411d2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -7568,6 +7568,11 @@ Kernel parameters
>  			See Documentation/admin-guide/mm/transhuge.rst for more
>  			details.
>  
> +	tlbi=		[X86-64]
> +			Format: {off}
> +			Control the AMD TLBI feature support (INVLPGB-based
> +			TLB flushing).
> +
>  	topology=	[S390,EARLY]
>  			Format: {off | on}
>  			Specify if the kernel should make use of the cpu
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 54e14ed276b5..a3cde941a81d 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1440,3 +1440,13 @@ static __init int print_dmi_agesa(void)
>  	return 0;
>  }
>  late_initcall(print_dmi_agesa);
> +
> +/* Control TLBI feature (INVLPGB-based TLB flushing) */
> +static int __init tlbi_setup(char *str)
> +{
> +	if (!strcmp(str, "off"))
> +		setup_clear_cpu_cap(X86_FEATURE_INVLPGB);
> +
> +	return 1;
> +}
> +__setup("tlbi=", tlbi_setup);

TLBI stands for Translation Lookaside Buffer Invalidation. So a command
line that says: 'tlbi=off' means no move invalidation *at*all*. This
seems like a very bad option.

Please rename.