Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Kirill A. Shutemov]

On Wed, Jun 29, 2022 at 07:29:13PM -0700, Andy Lutomirski wrote:
> 
> 
> On Tue, Jun 28, 2022, at 5:53 PM, Kirill A. Shutemov wrote:
> > On Tue, Jun 28, 2022 at 04:42:40PM -0700, Andy Lutomirski wrote:
> >> On 6/10/22 07:35, Kirill A. Shutemov wrote:
> >> 
> >> > +	/* Update CR3 to get LAM active */
> >> > +	switch_mm(current->mm, current->mm, current);
> >> 
> >> Can you at least justify this oddity?  When changing an LDT, we use a
> >> dedicated mechanism.  Is there a significant benefit to abusing switch_mm
> >> for this?
> >
> > I'm not sure I follow. LAM mode is set in CR3. switch_mm() has to handle
> > it anyway to context switch. Why do you consider it abuse?
> >
> >> 
> >> Also, why can't we enable LAM on a multithreaded process?  We can change an
> >> LDT, and the code isn't even particularly complicated.
> >
> > I reworked this in v4[1] and it allows multithreaded processes. Have you
> > got that version?
> >
> > Intel had issue with mail server, but I assumed it didn't affect my
> > patchset since I see it in the archive.
> >
> 
> I didn’t notice it. Not quite sure what the issue was. Could just be
> incompetence on my part.
> 
> I think that’s the right idea, except that I think you shouldn’t use
> switch_mm for this. Just update the LAM bits directly.   Once you read
> mm_cpumask, you should be guaranteed (see next paragraph) that, for each
> CPU that isn’t in the set, if it switches to the new mm, it will notice
> the new LAM.
> 
> I say “should be” because I think smp_wmb() is insufficient. You’re
> ordering a write with a subsequent read, which needs smp_mb().

I think it is better to put smp_mb() to make it explicit.

Does the fixup below look okay?

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 2d70d75e207f..8da54e7b6f98 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -367,4 +367,30 @@ static inline void __native_tlb_flush_global(unsigned long cr4)
 	native_write_cr4(cr4 ^ X86_CR4_PGE);
 	native_write_cr4(cr4);
 }
+
+#ifdef CONFIG_X86_64
+static inline u64 tlbstate_lam_cr3_mask(void)
+{
+	u64 lam = this_cpu_read(cpu_tlbstate.lam);
+
+	return lam << X86_CR3_LAM_U57_BIT;
+}
+
+static inline void set_tlbstate_lam_cr3_mask(u64 mask)
+{
+	this_cpu_write(cpu_tlbstate.lam, mask >> X86_CR3_LAM_U57_BIT);
+}
+
+#else
+
+static inline u64 tlbstate_lam_cr3_mask(void)
+{
+	return 0;
+}
+
+static inline void set_tlbstate_lam_cr3_mask(u64 mask)
+{
+}
+#endif
+
 #endif /* _ASM_X86_TLBFLUSH_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 427ebef3f64b..cd2b03fe94c4 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -745,15 +745,16 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
 static void enable_lam_func(void *mm)
 {
 	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
+	unsigned long lam_mask;
 
 	if (loaded_mm != mm)
 		return;
 
-	/* Counterpart of smp_wmb() in prctl_enable_tagged_addr() */
-	smp_rmb();
+	lam_mask = READ_ONCE(loaded_mm->context.lam_cr3_mask);
 
 	/* Update CR3 to get LAM active on the CPU */
-	switch_mm(loaded_mm, loaded_mm, current);
+	write_cr3(__read_cr3() | lam_mask);
+	set_tlbstate_lam_cr3_mask(lam_mask);
 }
 
 static bool lam_u48_allowed(void)
@@ -805,7 +806,7 @@ static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
 	}
 
 	/* Make lam_cr3_mask and untag_mask visible on other CPUs */
-	smp_wmb();
+	smp_mb();
 
 	on_each_cpu_mask(mm_cpumask(mm), enable_lam_func, mm, true);
 out:
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index c5c4f76329c2..d9a2acdae90f 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -486,31 +486,6 @@ void cr4_update_pce(void *ignored)
 static inline void cr4_update_pce_mm(struct mm_struct *mm) { }
 #endif
 
-#ifdef CONFIG_X86_64
-static inline u64 tlbstate_lam_cr3_mask(void)
-{
-	u64 lam = this_cpu_read(cpu_tlbstate.lam);
-
-	return lam << X86_CR3_LAM_U57_BIT;
-}
-
-static inline void set_tlbstate_lam_cr3_mask(u64 mask)
-{
-	this_cpu_write(cpu_tlbstate.lam, mask >> X86_CR3_LAM_U57_BIT);
-}
-
-#else
-
-static inline u64 tlbstate_lam_cr3_mask(void)
-{
-	return 0;
-}
-
-static inline void set_tlbstate_lam_cr3_mask(u64 mask)
-{
-}
-#endif
-
 void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 			struct task_struct *tsk)
 {
@@ -581,7 +556,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 	 * provides that full memory barrier and core serializing
 	 * instruction.
 	 */
-	if (real_prev == next && prev_lam == new_lam) {
+	if (real_prev == next) {
 		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
 			   next->context.ctx_id);
 
-- 
 Kirill A. Shutemov