linux-kernel - Re: [PATCH v2] arm64/sysreg: Fix GIC CDEOI instruction encoding

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Message-ID: <aOUsjRkq4DTcLujW@arm.com>
Date: Tue, 7 Oct 2025 16:06:53 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Sascha Bischoff <sascha.bischoff@....com>,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v2] arm64/sysreg: Fix GIC CDEOI instruction encoding

On Tue, Oct 07, 2025 at 12:26:00PM +0200, Lorenzo Pieralisi wrote:
> The GIC CDEOI system instruction requires the Rt field to be set to 0b11111
> otherwise the instruction behaviour becomes CONSTRAINED UNPREDICTABLE.
> 
> Currenly, its usage is encoded as a system register write, with a constant
> 0 value:
> 
> write_sysreg_s(0, GICV5_OP_GIC_CDEOI)
> 
> While compiling with GCC, the 0 constant value, through these asm
> constraints and modifiers ('x' modifier and 'Z' constraint combo):
> 
> asm volatile(__msr_s(r, "%x0") : : "rZ" (__val));
> 
> forces the compiler to issue the XZR register for the MSR operation (ie
> that corresponds to Rt == 0b11111) issuing the right instruction encoding.
> 
> Unfortunately LLVM does not yet understand that modifier/constraint
> combo so it ends up issuing a different register from XZR for the MSR
> source, which in turns means that it encodes the GIC CDEOI instruction
> wrongly and the instruction behaviour becomes CONSTRAINED UNPREDICTABLE
> that we must prevent.
> 
> Add a conditional to write_sysreg_s() macro that detects whether it
> is passed a constant 0 value and issues an MSR write with XZR as source
> register - explicitly doing what the asm modifier/constraint is meant to
> achieve through constraints/modifiers, fixing the LLVM compilation issue.
> 
> Fixes: 7ec80fb3f025 ("irqchip/gic-v5: Add GICv5 PPI support")
> Suggested-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Lorenzo Pieralisi <lpieralisi@...nel.org>
> Acked-by: Marc Zyngier <maz@...nel.org>
> Cc: stable@...r.kernel.org
> Cc: Sascha Bischoff <sascha.bischoff@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marc Zyngier <maz@...nel.org>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

(unless Will sends another pull request before -rc1, I'll pick this
patch shortly after)