linux-kernel - Re: [PATCH] arm64: Introduce ISAR6 CPU ID register

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Date:   Thu, 12 Dec 2019 14:46:33 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH] arm64: Introduce ISAR6 CPU ID register

On Thu, Dec 12, 2019 at 03:44:23PM +0530, Anshuman Khandual wrote:
> +#define ID_ISAR6_JSCVT_SHIFT		0
> +#define ID_ISAR6_DP_SHIFT		4
> +#define ID_ISAR6_FHM_SHIFT		8
> +#define ID_ISAR6_SB_SHIFT		12
> +#define ID_ISAR6_SPECRES_SHIFT		16
> +#define ID_ISAR6_BF16_SHIFT		20
> +#define ID_ISAR6_I8MM_SHIFT		24

> @@ -399,6 +399,7 @@ static const struct __ftr_reg_entry {
>  	ARM64_FTR_REG(SYS_ID_ISAR4_EL1, ftr_generic_32bits),
>  	ARM64_FTR_REG(SYS_ID_ISAR5_EL1, ftr_id_isar5),
>  	ARM64_FTR_REG(SYS_ID_MMFR4_EL1, ftr_id_mmfr4),

> +	ARM64_FTR_REG(SYS_ID_ISAR6_EL1, ftr_generic_32bits),

Using ftr_generic_32bits exposes the lowest-common-denominator for all
4-bit fields in the register, and I don't think that's the right thing
to do here, because:

* We have no idea what ID_ISAR6 bits [31:28] may mean in future.

* AFAICT, the instructions described by ID_ISAR6.SPECRES (from the
  ARMv8.0-PredInv extension) operate on the local PE and are not
  broadcast. To make those work as a guest expects, the host will need
  to do additional things (e.g. to preserve that illusion when a vCPU is
  migrated from one pCPU to another and back).

Given that, think we should add an explicit ftr_id_isar6 which only
exposes the fields that we're certain are safe to expose to a guest
(i.e. without SPECRES).

Thanks,
Mark.