Re: [PATCH 5/5] KVM: arm64: Handle AArch32 SPSR_{irq,abt,und,fiq} as RAZ/WI

From: Oliver Upton <oliver.upton@linux.dev>
To: Marc Zyngier <maz@kernel.org>
Cc: Miguel Luis <miguel.luis@oracle.com>,
	"kvmarm@lists.linux.dev" <kvmarm@lists.linux.dev>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Eric Auger <eric.auger@redhat.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>
Subject: Re: [PATCH 5/5] KVM: arm64: Handle AArch32 SPSR_{irq,abt,und,fiq} as RAZ/WI
Date: Wed, 25 Oct 2023 08:46:13 +0000	[thread overview]
Message-ID: <ZTjV1Q05nXXOOyVO@linux.dev> (raw)
In-Reply-To: <86h6mf3y3w.wl-maz@kernel.org>

On Wed, Oct 25, 2023 at 09:28:03AM +0100, Marc Zyngier wrote:
> On Wed, 25 Oct 2023 00:04:27 +0100, Oliver Upton <oliver.upton@linux.dev> wrote:
> > Correction (I wasn't thinking): RES0 behavior should be invariant, much
> > like the UNDEF behavior of the other AA32-specific registers.
> 
> I'm not sure what you're asking for exactly here, so let me explain my
> understanding of the architecture on this point, which is that the
> *32_EL2 registers are different in nature from the SPSR_* registers.

Damn, I still didn't manage to get my point across!

> IFAR32_EL2 and co are accessors for the equivalent AArch32 registers.
> If AArch32 isn't implemented, then these registers should UNDEF,
> because there is nothing to access at all.
> 
> The status of SPSR_* is more subtle: the AArch32 exception model is
> banked (irq, fiq, abt, und), and for each bank we have a triplet
> (LR_*, SP_*, SPSR_*), plus the extra R[8-12]_fiq. On taking an
> exception from AArch32 EL1 to AArch64 EL2, all the (LR_*, SP_*,
> R*_fiq) are stored as part of the AArch64 GPRs (X16-X30, see I_PYKVS).

Thanks. Yeah, I've got a pretty good handle on what's going on here.
What I really was trying to compare is the way these aliases into AA32
state are handled, and the annoying difference between the two sets.

IFAR32 and friends UNDEF unconditionally w/o AArch32, which I quite
like.

To your point, the SPSR_* accessors still trap even if AArch32 is not
implemented. I was suggesting in passing that it'd be nice if the
architecture alternatively allowed for these to read as RES0 (no trap)
if NV==1 and AArch32 is not implemented, which aligns with your change.

But after all...

> we will never see an AArch32-capable, NV-capable HW implementation
> ever, so this is all fairly academic.

None of this matters in the first place :)

-- 
Thanks,
Oliver

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