Messages in this thread | | | From | Andy Lutomirski <> | Subject | Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}() | Date | Fri, 29 May 2020 10:28:33 -0700 |
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> On May 28, 2020, at 2:34 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > 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;
Why xor? This seems extra confusing.
> + 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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