Re: [PATCHv8 00/11] Linear Address Masking enabling

[Kirill A. Shutemov]

On Mon, Sep 05, 2022 at 05:46:49PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 05, 2022 at 06:35:17PM +0300, Kirill A. Shutemov wrote:
> > What about something like this?
> > 
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 803241dfc473..868d2730884b 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -30,8 +30,10 @@ static inline bool pagefault_disabled(void);
> >   */
> >  #define untagged_addr(mm, addr)	({					\
> >  	u64 __addr = (__force u64)(addr);				\
> > -	s64 sign = (s64)__addr >> 63;					\
> > -	__addr &= (mm)->context.untag_mask | sign;			\
> > +	if (static_cpu_has(X86_FEATURE_LAM)) {				\
> > +		s64 sign = (s64)__addr >> 63;				\
> > +		__addr &= (mm)->context.untag_mask | sign;		\
> > +	}								\
> >  	(__force __typeof__(addr))__addr;				\
> >  })
> 
> Well, if you go throught the trouble of adding it, might as well use a
> regular static branch and only enable it once there's an actual user,
> no?

Fair enough. How about this?

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 803241dfc473..1a03c65a9c0f 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -22,6 +22,8 @@ static inline bool pagefault_disabled(void);
 #endif
 
 #ifdef CONFIG_X86_64
+DECLARE_STATIC_KEY_FALSE(tagged_addr_key);
+
 /*
  * Mask out tag bits from the address.
  *
@@ -30,8 +32,10 @@ static inline bool pagefault_disabled(void);
  */
 #define untagged_addr(mm, addr)	({					\
 	u64 __addr = (__force u64)(addr);				\
-	s64 sign = (s64)__addr >> 63;					\
-	__addr &= (mm)->context.untag_mask | sign;			\
+	if (static_branch_unlikely(&tagged_addr_key)) {			\
+		s64 sign = (s64)__addr >> 63;				\
+		__addr &= (mm)->context.untag_mask | sign;		\
+	}								\
 	(__force __typeof__(addr))__addr;				\
 })
 
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 337f80a0862f..63194bf43c9a 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -742,6 +742,9 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
 }
 #endif
 
+DEFINE_STATIC_KEY_FALSE(tagged_addr_key);
+EXPORT_SYMBOL_GPL(tagged_addr_key);
+
 static void enable_lam_func(void *mm)
 {
 	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
@@ -813,6 +816,7 @@ static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
 	}
 
 	on_each_cpu_mask(mm_cpumask(mm), enable_lam_func, mm, true);
+	static_branch_enable(&tagged_addr_key);
 out:
 	mutex_unlock(&mm->context.lock);
 	mmap_write_unlock(mm);
-- 
  Kiryl Shutsemau / Kirill A. Shutemov