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
