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

/*
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