Messages in this thread Patch in this message | | | Date | Thu, 28 May 2020 23:33:52 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}() |
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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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