linux-kernel - Re: [PATCH v4 2/5] arm64: initialise SCTLR2

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Message-ID: <aLXmCJOuxCHVXEYx@e129823.arm.com>
Date: Mon, 1 Sep 2025 19:29:28 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: catalin.marinas@....com, will@...nel.org, broonie@...nel.org,
	oliver.upton@...ux.dev, anshuman.khandual@....com, robh@...nel.org,
	james.morse@....com, mark.rutland@....com, joey.gouly@....com,
	ahmed.genidi@....com, kevin.brodsky@....com,
	scott@...amperecomputing.com, mbenes@...e.cz,
	james.clark@...aro.org, frederic@...nel.org, rafael@...nel.org,
	pavel@...nel.org, ryan.roberts@....com, suzuki.poulose@....com,
	maz@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v4 2/5] arm64: initialise SCTLR2_ELx register at boot time

Hi Dave,

> On Thu, Aug 21, 2025 at 06:24:05PM +0100, Yeoreum Yun wrote:
> > The value of the SCTLR2_ELx register is UNKNOWN after reset.
> > If the firmware initializes these registers properly, no additional
> > initialization is required.
> > However, in cases where they are not initialized correctly,
> > initialize the SCTLR2_ELx registers during CPU/vCPU boot
> > to prevent unexpected system behavior caused by invalid values.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> > ---
>
> [...]
>
> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index 23be85d93348..c25c2aed5125 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -738,6 +738,21 @@ alternative_endif
> >  	set_sctlr sctlr_el2, \reg
> >  .endm
> >
> > +/* Set SCTLR2_ELx to the @reg value. */
> > +.macro set_sctlr2_elx, el, reg, tmp
> > +	mrs_s	\tmp, SYS_ID_AA64MMFR3_EL1
> > +	ubfx	\tmp, \tmp, #ID_AA64MMFR3_EL1_SCTLRX_SHIFT, #4
> > +	cbz	\tmp, .Lskip_sctlr2_\@
> > +	.if	\el == 2
> > +	msr_s	SYS_SCTLR2_EL2, \reg
> > +	.elseif	\el == 12
> > +	msr_s	SYS_SCTLR2_EL12, \reg
> > +	.else
> > +	msr_s	SYS_SCTLR2_EL1, \reg
> > +	.endif
> > +.Lskip_sctlr2_\@:
> > +.endm
> > +
>
> You were right that just doing
>
> 	msr_s	SYS_SCTLR_\el, \reg
>
> here doesn't work.  It looks like we rely on the C preprocessor to
> expand the SYS_FOO_REG names to numeric sysreg encodings.  By the time
> the assembler macros are expanded, it is too late to construct sysreg
> names -- the C preprocessor only runs once, before the assembler.
>
> So, your code here looks reasonable.
>
> But, it will still silently do the wrong thing if \el is empty or
> garbage, so it is probably worth adding an error check:
>
> 	.else
> 	.error "Bad EL \"\el\" in set_sctlr2_elx"
> 	.endif
>
>
> Also, looking at all this again, the "1", "2" and "12" suffixes are not
> really numbers: SYS_REG_EL02 would definitely not be the same thing as
> SYS_REG_EL2 (although there is no _EL02 version of this particular
> register).
>
> So, can you use .ifc to do a string comparison instead?
>
> If might be a good idea to migrate other macros that use an "el"
> argument to do something similar -- although that probably doesn't
> belong in this series.
>
> The assembler seems to have no ".elseifc" directive, so you'll need
> separate nested .ifc blocks:
>
> 	.ifc	\el,2
> 	msr_s	SYS_SCTLR2_EL2, \reg
> 	.else
> 	.ifc	\el,12
> 	msr_s	SYS_SCTLR2_EL12, \reg
> 	.else
> 	.ifc	\el,1
> 	msr_s	SYS_SCTLR2_EL1, \reg
> 	.else
> 	.error	"Bad EL \"\el\" in set_sctlr2_elx"
> 	.endif
> 	.endif
> 	.endif
>
> (Note, I've not tested this approach.  If you can think of a better
> way, please feel free to suggest.)

Thanks for your suggestion. Let me test and check about this idea could
be applied in other macro too :D
(But as you mention, I'll apply this for SCTLR2 in other patchset).

>
> [...]
>
> > diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> > index 36e2d26b54f5..ac12f1b4f8e2 100644
> > --- a/arch/arm64/kernel/hyp-stub.S
> > +++ b/arch/arm64/kernel/hyp-stub.S
> > @@ -144,7 +144,17 @@ SYM_CODE_START_LOCAL(__finalise_el2)
> >
> >  .Lskip_indirection:
> >  .Lskip_tcr2:
> > +	mrs_s	x1, SYS_ID_AA64MMFR3_EL1
> > +	ubfx	x1, x1, #ID_AA64MMFR3_EL1_SCTLRX_SHIFT, #4
> > +	cbz	x1, .Lskip_sctlr2
> > +	mrs_s	x1, SYS_SCTLR2_EL12
> > +	msr_s	SYS_SCTLR2_EL1, x1
> >
> > +	// clean SCTLR2_EL1
> > +	mov_q	x1, INIT_SCTLR2_EL1
> > +	msr_s	SYS_SCTLR2_EL12, x1
>
> I'm still not sure why we need to do this.  The code doesn't seem to
> clean up by the EL1 value of any other register -- or have I missed
> something?
>
> We have already switched to EL2, via the HVC call that jumped to
> __finalise_el2.  We won't run at EL1 again unless KVM starts a guest --
> but in that case, it's KVM's responsibility to set up the EL1 registers
> before handing control to the guest.
>
> In any case, is SCTLR2_EL1 ever set to anything except INIT_SCTLR2_EL1
> before we get here?

Regardless of VHE and nVHE, between init_kernel_el() and finalise_el2()
the kernel modifies SCTLR_EL1/SCTLR2_EL1 (since el level in this period
is EL1).
and at the end of finalise_el2(), kernel switches to el2 and
if VHE, it writes the SCTLR_EL1/SCTLR2_EL1 to SCTLR_EL2/SCTLR2_EL2 and
initialize the SCTLR_EL1/SCTLR2_EL1.

Although there is no code to modify SCTLR2_EL1 between this period,
as SCTLR1_ELx, I initialize the SCTLR2_EL1 as init value for the future
usage.

Thanks!

--
Sincerely,
Yeoreum Yun