linux-kernel - Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change ID_AA64PFR1

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Date: Tue, 18 Jun 2024 08:39:32 +0100
From: Marc Zyngier <maz@...nel.org>
To: Shaoqin Huang <shahuang@...hat.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	kvmarm@...ts.linux.dev,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1

On Tue, 18 Jun 2024 07:38:06 +0100,
Shaoqin Huang <shahuang@...hat.com> wrote:
> 
> Allow userspace to change the guest-visible value of the register with
> some severe limitation:
> 
>   - No changes to features not virtualized by KVM (MPAM_frac, RAS_frac)
> ---
>  arch/arm64/kvm/sys_regs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 22b45a15d068..bead81867bce 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2306,7 +2306,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  		   ID_AA64PFR0_EL1_GIC |
>  		   ID_AA64PFR0_EL1_AdvSIMD |
>  		   ID_AA64PFR0_EL1_FP), },
> -	ID_SANITISED(ID_AA64PFR1_EL1),
> +	ID_WRITABLE(ID_AA64PFR1_EL1, ~(ID_AA64PFR1_EL1_RAS_frac |
> +				       ID_AA64PFR1_EL1_MPAM_frac)),
>  	ID_UNALLOCATED(4,2),
>  	ID_UNALLOCATED(4,3),
>  	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),

This isn't a valid patch.

Furthermore, how about all the other features that may or may not be
currently handled by KVM? Please see [1] and make sure that all
existing fields have a known behaviour (a combination of masked,
preserved, capped, writable or read-only).

I can at least see problems with MTE_frac and MTEX, plus all the other
things that KVM doesn't know how to save/restore (THE, GCS, NMI...).

What I asked you to handle the whole register, I really meant it.

	M.

[1] https://developer.arm.com/documentation/ddi0601/2024-03/AArch64-Registers/ID-AA64PFR1-EL1--AArch64-Processor-Feature-Register-1?lang=en

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