Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

["Aiqun(Maria) Yu" <quic_aiquny@xxxxxxxxxxx>]

On 7/10/2023 3:27 PM, Marc Zyngier wrote:
> On Mon, 10 Jul 2023 06:59:55 +0100,
> Maria Yu <quic_aiquny@xxxxxxxxxxx> wrote:
> > In order to be able to disable lse_atomic even if cpu
> > support it, most likely because of memory controller
> > cannot deal with the lse atomic instructions, use a
> > new idreg override to deal with it.
>
> In general, the idreg overrides are *not* there to paper over HW bugs.
> They are there to force the kernel to use or disable a feature for
> performance reason or to guide the *enabling* of a feature, but not
> because the HW is broken.
>
> The broken status of a HW platform must also be documented so that we
> know what to expect when we look at, for example, a bad case of memory
> corruption (something I'd expect to see on a system that only
> partially implements atomic memory operations).

good idea. A noc error would be happened if the lse atomic instruction 
happened during a memory controller doesn't support lse atomic instructions.
I can put the information in next patchset comment message. Pls feel 
free to let know if there is other place to have this kind of 
information with.
> > Signed-off-by: Maria Yu <quic_aiquny@xxxxxxxxxxx>
> > ---
> >   Documentation/admin-guide/kernel-parameters.txt |  2 ++
> >   arch/arm64/include/asm/cpufeature.h             |  1 +
> >   arch/arm64/kernel/cpufeature.c                  |  4 +++-
> >   arch/arm64/kernel/idreg-override.c              | 11 +++++++++++
> >   4 files changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 85fb0fa5d091..6ad754549f1d 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -433,6 +433,8 @@
> >   	arm64.nomops	[ARM64] Unconditionally disable Memory Copy and Memory
> >   			Set instructions support
> >   
> > +	arm64.nolse_atomic [ARM64] Unconditionally disable LSE Atomic support
> > +
>
> 'nolse', or 'noatomic' should be enough. In general, the suffix should
> be either derived from the FEAT_* name or the idreg field name.

noatomic can be used in next patchset.
ID_AA64ISAR0_EL1_ATOMIC_SHIFT
> >   	ataflop=	[HW,M68k]
> >   
> >   	atarimouse=	[HW,MOUSE] Atari Mouse
> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index 96e50227f940..9d56dea1fe62 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -916,6 +916,7 @@ extern struct arm64_ftr_override id_aa64pfr0_override;
> >   extern struct arm64_ftr_override id_aa64pfr1_override;
> >   extern struct arm64_ftr_override id_aa64zfr0_override;
> >   extern struct arm64_ftr_override id_aa64smfr0_override;
> > +extern struct arm64_ftr_override id_aa64isar0_override;
> >   extern struct arm64_ftr_override id_aa64isar1_override;
> >   extern struct arm64_ftr_override id_aa64isar2_override;
> >   
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index f9d456fe132d..9bd766880807 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -669,6 +669,7 @@ struct arm64_ftr_override __ro_after_init id_aa64pfr0_override;
> >   struct arm64_ftr_override __ro_after_init id_aa64pfr1_override;
> >   struct arm64_ftr_override __ro_after_init id_aa64zfr0_override;
> >   struct arm64_ftr_override __ro_after_init id_aa64smfr0_override;
> > +struct arm64_ftr_override __ro_after_init id_aa64isar0_override;
> >   struct arm64_ftr_override __ro_after_init id_aa64isar1_override;
> >   struct arm64_ftr_override __ro_after_init id_aa64isar2_override;
> >   
> > @@ -721,7 +722,8 @@ static const struct __ftr_reg_entry {
> >   	ARM64_FTR_REG(SYS_ID_AA64DFR1_EL1, ftr_raz),
> >   
> >   	/* Op1 = 0, CRn = 0, CRm = 6 */
> > -	ARM64_FTR_REG(SYS_ID_AA64ISAR0_EL1, ftr_id_aa64isar0),
> > +	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64ISAR0_EL1, ftr_id_aa64isar0,
> > +			       &id_aa64isar0_override),
> >   	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64ISAR1_EL1, ftr_id_aa64isar1,
> >   			       &id_aa64isar1_override),
> >   	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64ISAR2_EL1, ftr_id_aa64isar2,
> > diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
> > index 2fe2491b692c..af41ab4f3d94 100644
> > --- a/arch/arm64/kernel/idreg-override.c
> > +++ b/arch/arm64/kernel/idreg-override.c
> > @@ -105,6 +105,15 @@ static const struct ftr_set_desc pfr1 __initconst = {
> >   	},
> >   };
> >   
> > +static const struct ftr_set_desc isar0 __initconst = {
> > +	.name		= "id_aa64isar0",
> > +	.override	= &id_aa64isar0_override,
> > +	.fields		= {
> > +	        FIELD("atomic", ID_AA64ISAR0_EL1_ATOMIC_SHIFT, NULL),
> > +		{}
> > +	},
> > +};
> > +
> >   static const struct ftr_set_desc isar1 __initconst = {
> >   	.name		= "id_aa64isar1",
> >   	.override	= &id_aa64isar1_override,
> > @@ -163,6 +172,7 @@ static const struct ftr_set_desc * const regs[] __initconst = {
> >   	&mmfr1,
> >   	&pfr0,
> >   	&pfr1,
> > +	&isar0,
> >   	&isar1,
> >   	&isar2,
> >   	&smfr0,
> > @@ -185,6 +195,7 @@ static const struct {
> >   	{ "arm64.nomops",		"id_aa64isar2.mops=0" },
> >   	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
> >   	{ "nokaslr",			"arm64_sw.nokaslr=1" },
> > +	{ "arm64.nolse_atomic",         "id_aa64isar0.atomic=0" },
>
> And what of 32bit?
>
> 	M.

--
Thx and BRs,
Aiqun(Maria) Yu