Re: [PATCH v5 4/4] arm64: errata: Work around early CME DVMSync acknowledgement

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Apr 10, 2026 at 01:09:41PM +0100, Will Deacon wrote:
> On Tue, Apr 07, 2026 at 11:28:44AM +0100, Catalin Marinas wrote:
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 489554931231..4c328b7c79ba 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -26,6 +26,7 @@
> >  #include <linux/reboot.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/init.h>
> > +#include <linux/cpumask.h>
> >  #include <linux/cpu.h>
> >  #include <linux/elfcore.h>
> >  #include <linux/pm.h>
> > @@ -339,8 +340,41 @@ void flush_thread(void)
> >  	flush_gcs();
> >  }
> >  
> > +#ifdef CONFIG_ARM64_ERRATUM_4193714
> > +
> > +static void arch_dup_tlbbatch_mask(struct task_struct *dst)
> > +{
> > +	/*
> > +	 * Clear the inherited cpumask with memset() to cover both cases where
> > +	 * cpumask_var_t is a pointer or an array. It will be allocated lazily
> > +	 * in sme_dvmsync_add_pending() if CPUMASK_OFFSTACK=y.
> > +	 */
> > +	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_4193714))
> > +		memset(&dst->tlb_ubc.arch.cpumask, 0,
> > +		       sizeof(dst->tlb_ubc.arch.cpumask));
> 
> nit: use cpumask_clear() instead?

I tried to explain in the comment above. The memset() is on purpose to
avoid #ifdef CPUMASK_OFFSTACK. When enabled, cpumask_var_t is a pointer
and we want it set to NULL (for later lazy allocation) rather than
clearing the parent's cpumask. I had the diff below initially but it
looked uglier. Or we make the erratum dependent on !CPUMASK_OFFSTACK but
it won't get compile coverage with defconfig.

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 4c328b7c79ba..9294cbd35cce 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -345,13 +345,18 @@ void flush_thread(void)
 static void arch_dup_tlbbatch_mask(struct task_struct *dst)
 {
 	/*
-	 * Clear the inherited cpumask with memset() to cover both cases where
-	 * cpumask_var_t is a pointer or an array. It will be allocated lazily
-	 * in sme_dvmsync_add_pending() if CPUMASK_OFFSTACK=y.
+	 * Don't inherit the parent's tlbbatch cpumask.
+	 *
+	 * With CPUMASK_OFFSTACK=y, cpumask_var_t is a pointer. Reset it so
+	 * that it will be allocated lazily in sme_dvmsync_add_pending().
 	 */
-	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_4193714))
-		memset(&dst->tlb_ubc.arch.cpumask, 0,
-		       sizeof(dst->tlb_ubc.arch.cpumask));
+	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_4193714)) {
+#ifdef CONFIG_CPUMASK_OFFSTACK
+		dst->tlb_ubc.arch.cpumask = NULL;
+#else
+		cpumask_clear(dst->tlb_ubc.arch.cpumask);
+#endif
+	}
 }

 static void arch_release_tlbbatch_mask(struct task_struct *tsk)

-- 
Catalin