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

[Peter Zijlstra]

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.

> 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 ;-)