Message-ID: <393a03b2b681d12edfd864b3d02a7378611a4bcf.camel@linux.ibm.com>
Subject: Re: [RFC PATCH 02/13] powerpc: Add initial Dynamic Execution
 Control Register (DEXCR) support
From:   Benjamin Gray <bgray@linux.ibm.com>
To:     Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
Cc:     linux-hardening@vger.kernel.org, ajd@linux.ibm.com,
        cmr@bluescreens.de, linux-kernel@vger.kernel.org
Date:   Fri, 10 Mar 2023 10:46:51 +1100
In-Reply-To: <CQZVLQQBO6K9.2A71BY640ZH5P@bobo>
References: <20221128024458.46121-1-bgray@linux.ibm.com>
         <20221128024458.46121-3-bgray@linux.ibm.com>
         <CQZVLQQBO6K9.2A71BY640ZH5P@bobo>
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On Tue, 2023-03-07 at 14:45 +1000, Nicholas Piggin wrote:
> On Mon Nov 28, 2022 at 12:44 PM AEST, Benjamin Gray wrote:
> > diff --git a/arch/powerpc/include/asm/cputable.h
> > b/arch/powerpc/include/asm/cputable.h
> > index 757dbded11dc..03bc192f2d8b 100644
> > --- a/arch/powerpc/include/asm/cputable.h
> > +++ b/arch/powerpc/include/asm/cputable.h
> > @@ -192,6 +192,10 @@ static inline void cpu_feature_keys_init(void)
> > { }
> > =C2=A0#define
> > CPU_FTR_P9_RADIX_PREFETCH_BUG=C2=A0=C2=A0LONG_ASM_CONST(0x0002000000000=
000)
> > =C2=A0#define
> > CPU_FTR_ARCH_31=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0LONG_ASM_CONST(0x00040000000
> > 00000)
> > =C2=A0#define
> > CPU_FTR_DAWR1=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0LONG_ASM_CONST(0x0008000=
000000000)
> > +#define
> > CPU_FTR_DEXCR_SBHE=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0LONG_ASM_CONST(0x0010000000000000)
> > +#define
> > CPU_FTR_DEXCR_IBRTPD=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0LONG_ASM_CONST(0x0020000000000000)
> > +#define
> > CPU_FTR_DEXCR_SRAPD=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0LONG_ASM_CONST(0x0040000000000000)
> > +#define
> > CPU_FTR_DEXCR_NPHIE=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0LONG_ASM_CONST(0x0080000000000000)
>=20
> We potentially don't need to use CPU_FTR bits for each of these. We
> only really want them to use instruction patching and make feature
> tests fast. But we have been a bit liberal with using them and they
> are kind of tied into cpu feature parsing code so maybe it's easier
> to go with them for now.

For the static only DEXCR series I've only got CPU_FTR_DEXCR_NPHIE
because that's needed for hashkey updates. The others don't really
matter; they are only interesting for masking out unsupported bits.
Masking itself seems to be unnecessary; the DEXCR will just ignore
unsupported bits. Attempting to set all bits on a P10 showed the first
8 were set and the remainder stayed 0'd, and the kernel worked fine.

It's definitely easier to use CPU_FTR_* for feature detection from the
PAPR specified blob though. Maybe it would be possible to support a
callback on a match instead of setting a feature flag.
@@ -1802,7 +1809,7 @@ int copy_thread(struct task_struct *p, const
>=20
>=20

> > @@ -1802,7 +1809,7 @@ int copy_thread(struct task_struct *p, const
> > struct kernel_clone_args *args)
> > =C2=A0
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0setup_ksp_vsid(p, sp);
> > =C2=A0
> > -#ifdef CONFIG_PPC64=20
> > +#ifdef CONFIG_PPC64
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (cpu_has_feature(CPU=
_FTR_DSCR)) {
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0p->thread.dscr_inherit =3D current-
> > >thread.dscr_inherit;
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0p->thread.dscr =3D mfspr(SPRN_DSCR);
> > @@ -1939,6 +1946,10 @@ void start_thread(struct pt_regs *regs,
> > unsigned long start, unsigned long sp)
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0current->thread.tm_tfia=
r =3D 0;
> > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0current->thread.load_tm=
 =3D 0;
> > =C2=A0#endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
> > +#ifdef CONFIG_PPC_BOOK3S_64
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0if (cpu_has_feature(CPU_FTR_=
ARCH_31))
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0mtspr(SPRN_DEXCR, get_thread_dexcr(&current-
> > >thread));
> > +#endif /* CONFIG_PPC_BOOK3S_64 */
>=20
> You possibly don't need the ifdef here because CPU_FTR_ARCH_31 should
> fold away. Some of the others do because they're using open-coded
> access to struct members, but if you're using accessor functions to
> get and set such things, there may be no need to.
>=20
> I think my preference is for your style.

I've been revisiting where the DEXCR is initialised and updated. With
the static DEXCR, the thread value is just a field on the task struct
like the others.