linux-kernel - Re: [PATCH v4 14/21] arm64: Honor VHE being disabled from the command-line

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Message-ID: <87wnw2jph0.wl-maz@kernel.org>
Date:   Sun, 24 Jan 2021 15:59:07 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Jing Zhang <jingzhangos@...gle.com>,
        Ajay Patil <pajay@....qualcomm.com>,
        Prasad Sodagudi <psodagud@...eaurora.org>,
        Srinivas Ramana <sramana@...eaurora.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 14/21] arm64: Honor VHE being disabled from the command-line

On Sat, 23 Jan 2021 14:07:53 +0000,
Catalin Marinas <catalin.marinas@....com> wrote:
> 
> On Mon, Jan 18, 2021 at 09:45:26AM +0000, Marc Zyngier wrote:
> > diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> > index 59820f9b8522..bbab2148a2a2 100644
> > --- a/arch/arm64/kernel/hyp-stub.S
> > +++ b/arch/arm64/kernel/hyp-stub.S
> > @@ -77,13 +77,24 @@ SYM_CODE_END(el1_sync)
> >  SYM_CODE_START_LOCAL(mutate_to_vhe)
> >  	// Sanity check: MMU *must* be off
> >  	mrs	x0, sctlr_el2
> > -	tbnz	x0, #0, 1f
> > +	tbnz	x0, #0, 2f
> >  
> >  	// Needs to be VHE capable, obviously
> >  	mrs	x0, id_aa64mmfr1_el1
> >  	ubfx	x0, x0, #ID_AA64MMFR1_VHE_SHIFT, #4
> > -	cbz	x0, 1f
> > +	cbz	x0, 2f
> >  
> > +	// Check whether VHE is disabled from the command line
> > +	adr_l	x1, id_aa64mmfr1_val
> > +	ldr	x0, [x1]
> > +	adr_l	x1, id_aa64mmfr1_mask
> > +	ldr	x1, [x1]
> > +	ubfx	x0, x0, #ID_AA64MMFR1_VHE_SHIFT, #4
> > +	ubfx	x1, x1, #ID_AA64MMFR1_VHE_SHIFT, #4
> > +	cbz	x1, 1f
> > +	and	x0, x0, x1
> > +	cbz	x0, 2f
> > +1:
> 
> I can see the advantage here in separate id_aa64mmfr1_val/mask but we
> could use some asm offsets here and keep the pointer indirection simpler
> in C code. You'd just need something like 'adr_l mmfr1_ovrd + VAL_OFFSET'.
> 
> Anyway, if you have a strong preference for the current approach, leave
> it as is.

I've now moved over to a structure containing both val/mask, meaning
that we only need to keep a single pointer around in the various
feature descriptors. It certainly looks better.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.