linux-kernel - RE: [PATCH v2 07/11] arm64: mops: handle MOPS exceptions

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Date:   Thu, 25 May 2023 19:50:49 +0000
From:   Colton Lewis <coltonlewis@...gle.com>
To:     Kristina Martsenko <kristina.martsenko@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Luis Machado <luis.machado@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        linux-kernel@...r.kernel.org
Subject: RE: [PATCH v2 07/11] arm64: mops: handle MOPS exceptions

> +	if (esr & ESR_ELx_MOPS_ISS_MEM_INST) {
> +		/* SET* instruction */
> +		if (option_a ^ wrong_option) {
> +			/* Format is from Option A; forward set */
> +			pt_regs_write_reg(regs, dstreg, dst + size);
> +			pt_regs_write_reg(regs, sizereg, -size);
> +		}
> +	} else {
> +		/* CPY* instruction */
> +		if (!(option_a ^ wrong_option)) {
> +			/* Format is from Option B */
> +			if (regs->pstate & PSR_N_BIT) {
> +				/* Backward copy */
> +				pt_regs_write_reg(regs, dstreg, dst - size);
> +				pt_regs_write_reg(regs, srcreg, src - size);
> +			}
> +		} else {
> +			/* Format is from Option A */
> +			if (size & BIT(63)) {
> +				/* Forward copy */
> +				pt_regs_write_reg(regs, dstreg, dst + size);
> +				pt_regs_write_reg(regs, srcreg, src + size);
> +				pt_regs_write_reg(regs, sizereg, -size);
> +			}
> +		}
> +	}

I can see an argument for styling things closely to the ARM manual as
you have done here, but Linux style recommends against deep nesting. In
this case it is unneeded. I believe this can be written as a single
if-else chain and that makes it easier to distinguish the three options.

if ((esr & ESR_ELx_MOPS_ISS_MEM_INST) && (option_a ^ wrong_option)) {
	/* Format is from Option A; forward set */
	pt_regs_write_reg(regs, dstreg, dst + size);
	pt_regs_write_reg(regs, sizereg, -size);
} else if ((option_a ^ wrong_option) && (size & BIT(63)) {
	/* Forward copy */
	pt_regs_write_reg(regs, dstreg, dst + size);
	pt_regs_write_reg(regs, srcreg, src + size);
	pt_regs_write_reg(regs, sizereg, -size);
} else if (regs-pstate & PSR_N_BIT) {
	/* Backward copy */
	pt_regs_write_reg(regs, dstreg, dst - size);
	pt_regs_write_reg(regs, srcreg, src - size);
}