Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 28, 2020 at 11:15:50PM +0200, Peter Zijlstra wrote:
> On Thu, May 28, 2020 at 09:52:30PM +0100, Andrew Cooper wrote:
> > On 28/05/2020 21:19, Peter Zijlstra wrote:
> > > --- a/arch/x86/include/asm/debugreg.h
> > > +++ b/arch/x86/include/asm/debugreg.h
> > > @@ -113,6 +113,31 @@ static inline void debug_stack_usage_inc
> > >  static inline void debug_stack_usage_dec(void) { }
> > >  #endif /* X86_64 */
> > >  
> > > +static __always_inline void local_db_save(unsigned long *dr7)
> > > +{
> > > +	get_debugreg(*dr7, 7);
> > > +	if (*dr7)
> > > +		set_debugreg(0, 7);
> > 
> > %dr7 has an architecturally stuck bit in it.
> > 
> > You want *dr7 != 0x400 to avoid writing 0 unconditionally.
> 
> Do we have to have that bit set when writing it? Otherwise I might
> actually prefer masking it out.

I'm an idiot, we write a plain 9..

> > Also, API wise, wouldn't it be nicer to write "dr7 = local_db_save()"
> > rather than having a void function returning a single long via pointer?
> 
> Probably.. I started with local_irq_save() and .. well, n/m. I'll change
> it ;-)

How's this?

---
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -113,6 +113,36 @@ static inline void debug_stack_usage_inc
 static inline void debug_stack_usage_dec(void) { }
 #endif /* X86_64 */
 
+static __always_inline unsigned long local_db_save(void)
+{
+	unsigned long dr7;
+
+	get_debugreg(&dr7, 7);
+	dr7 ^= 0x400;
+	if (dr7)
+		set_debugreg(0, 7);
+	/*
+	 * Ensure the compiler doesn't lower the above statements into
+	 * the critical section; disabling breakpoints late would not
+	 * be good.
+	 */
+	barrier();
+
+	return dr7;
+}
+
+static __always_inline void local_db_restore(unsigned long dr7)
+{
+	/*
+	 * Ensure the compiler doesn't raise this statement into
+	 * the critical section; enabling breakpoints early would
+	 * not be good.
+	 */
+	barrier();
+	if (dr7)
+		set_debugreg(dr7, 7);
+}
+
 #ifdef CONFIG_CPU_SUP_AMD
 extern void set_dr_addr_mask(unsigned long mask, int dr);
 #else
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -727,15 +727,7 @@ static __always_inline void debug_enter(
 	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which
 	 * includes the entry stack is excluded for everything.
 	 */
-	get_debugreg(*dr7, 7);
-	set_debugreg(0, 7);
-
-	/*
-	 * Ensure the compiler doesn't lower the above statements into
-	 * the critical section; disabling breakpoints late would not
-	 * be good.
-	 */
-	barrier();
+	*dr7 = local_db_save();
 
 	/*
 	 * The Intel SDM says:
@@ -756,13 +748,7 @@ static __always_inline void debug_enter(
 
 static __always_inline void debug_exit(unsigned long dr7)
 {
-	/*
-	 * Ensure the compiler doesn't raise this statement into
-	 * the critical section; enabling breakpoints early would
-	 * not be good.
-	 */
-	barrier();
-	set_debugreg(dr7, 7);
+	local_db_restore(dr7);
 }
 
 /*