Re: [PATCH v2] powerpc xmon: use `dcbf` inplace of `dcbi` instruction for 64bit Book3S

[Balamuruhan S <bala24@linux.ibm.com>]

On Fri, 2020-03-27 at 07:48 +0100, Christophe Leroy wrote:
> 
> Le 26/03/2020 à 07:15, Balamuruhan S a écrit :
> > Data Cache Block Invalidate (dcbi) instruction was implemented back in
> > PowerPC
> > architecture version 2.03. It is obsolete and attempt to use of this
> > illegal
> > instruction results in a hypervisor emulation assistance interrupt. So,
> > ifdef
> > it out the option `i` in xmon for 64bit Book3S.
> 
> I don't understand. You say two contradictory things:
> 1/ You say it _was_ added back.
> 2/ You say it _is_ obsolete.
> 
> How can it be obsolete if it was added back ?

I actually learnt it from P8 and P9 User Manual,

The POWER8/POWER9 core does not provide support for the following optional or
obsolete instructions (attempted use of these results in a hypervisor emulation
assistance interrupt):
• tlbia - TLB invalidate all
• tlbiex - TLB invalidate entry by index (obsolete)
• slbiex - SLB invalidate entry by index (obsolete)
• dcba - Data cache block allocate (Book II; obsolete)
• dcbi - Data cache block invalidate (obsolete)
• rfi - Return from interrupt (32-bit; obsolete)

> 
> [...]
> 
> > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> > index 0ec9640335bb..bfd5a97689cd 100644
> > --- a/arch/powerpc/xmon/xmon.c
> > +++ b/arch/powerpc/xmon/xmon.c
> > @@ -335,10 +335,12 @@ static inline void cflush(void *p)
> >   	asm volatile ("dcbf 0,%0; icbi 0,%0" : : "r" (p));
> >   }
> >   
> > +#ifndef CONFIG_PPC_BOOK3S_64
> 
> You don't need that #ifndef. Keeping it should be harmless.

okay.

> 
> >   static inline void cinval(void *p)
> >   {
> >   	asm volatile ("dcbi 0,%0; icbi 0,%0" : : "r" (p));
> >   }
> > +#endif
> >   
> >   /**
> >    * write_ciabr() - write the CIABR SPR
> > @@ -1791,8 +1793,9 @@ static void prregs(struct pt_regs *fp)
> >   
> >   static void cacheflush(void)
> >   {
> > -	int cmd;
> >   	unsigned long nflush;
> > +#ifndef CONFIG_PPC_BOOK3S_64
> 
> Don't make it so complex, see below
> 
> > +	int cmd;
> >   
> >   	cmd = inchar();
> >   	if (cmd != 'i')
> > @@ -1800,13 +1803,14 @@ static void cacheflush(void)
> >   	scanhex((void *)&adrs);
> >   	if (termch != '\n')
> >   		termch = 0;
> > +#endif
> >   	nflush = 1;
> >   	scanhex(&nflush);
> >   	nflush = (nflush + L1_CACHE_BYTES - 1) / L1_CACHE_BYTES;
> >   	if (setjmp(bus_error_jmp) == 0) {
> >   		catch_memory_errors = 1;
> >   		sync();
> > -
> > +#ifndef CONFIG_PPC_BOOK3S_64
> 
> You don't need that ifndef, just ensure below that regardless of cmd, 
> book3s/64 calls cflush and not cinval.
> 
> >   		if (cmd != 'i') {
> 
> The only thing you have to do is to replace the above test by:
> 
> 		if (cmd != 'i' || IS_ENABLED(CONFIG_PPC_BOOK3S_64)) {

yes, this is the better way.

> 
> >   			for (; nflush > 0; --nflush, adrs += L1_CACHE_BYTES)
> >   				cflush((void *) adrs);
> > @@ -1814,6 +1818,10 @@ static void cacheflush(void)
> >   			for (; nflush > 0; --nflush, adrs += L1_CACHE_BYTES)
> >   				cinval((void *) adrs);
> >   		}
> > +#else
> 
> Don't need that at all, it's a duplication of the above.

sure :+1:

Thanks for reviewing.

-- Bala

> 
> > +		for (; nflush > 0; --nflush, adrs += L1_CACHE_BYTES)
> > +			cflush((void *)adrs);
> > +#endif
> >   		sync();
> >   		/* wait a little while to see if we get a machine check */
> >   		__delay(200);
> > 
> > base-commit: a87b93bdf800a4d7a42d95683624a4516e516b4f
> > 
> 
> Christophe