linux-kernel - Re: powerpc: Add an inline function to update HID0

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Message-ID: <20150804105747.GA19966@in.ibm.com>
Date:	Tue, 4 Aug 2015 16:27:47 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	mikey@...ling.org
Subject: Re: powerpc: Add an inline function to update HID0

Hi Michael,

On Tue, Aug 04, 2015 at 08:08:58PM +1000, Michael Ellerman wrote:
> On Tue, 2015-04-08 at 08:30:58 UTC, "Gautham R. Shenoy" wrote:
> > Section 3.7 of Version 1.2 of the Power8 Processor User's Manual
> > prescribes that updates to HID0 be preceded by a SYNC instruction and
> > followed by an ISYNC instruction (Page 91).
> > 
> > Create a function name update_hid0() which follows this recipe and
> > invoke it from the static split core path.
> > 
> > Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/include/asm/kvm_ppc.h       | 11 +++++++++++
> 
> Why is it in there? It's not KVM related per se.

Ok. Will fix this.
> 
> Where should it go? I think reg.h would be best, ideally near the definition
> for HID0, though that's probably not possible because of ASSEMBLY requirements.
> So at the bottom of reg.h ?
> 
> > diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> > index c6ef05b..325f1d6 100644
> > --- a/arch/powerpc/include/asm/kvm_ppc.h
> > +++ b/arch/powerpc/include/asm/kvm_ppc.h
> > @@ -685,4 +685,15 @@ static inline ulong kvmppc_get_ea_indexed(struct kvm_vcpu *vcpu, int ra, int rb)
> >  
> >  extern void xics_wake_cpu(int cpu);
> >  
> > +static inline void update_hid0(unsigned long hid0)
> > +{
> > +	/*
> > +	 *  The HID0 update should at the very least be preceded by a
> > +	 *  a SYNC instruction followed by an ISYNC instruction
> > +	 */
> > +	mb();
> > +	mtspr(SPRN_HID0, hid0);
> > +	isync();
> 
> That's going to turn into three separate inline asm blocks, which is maybe a
> bit unfortunate. Have you checked the generated code is what we want, ie. just
> sync, mtspr, isync ?
> 

Yes, the objdump of subcore.o shows exactly these three instructions:
 7c 00 04 ac     sync    
 7c 70 fb a6     mtspr   1008,r3
 4c 00 01 2c     isync

> cheers
>
--
Thanks and Regards
gautham. 

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