Re: [PATCH 3/4] KVM: arm64: show writable masks for feature registers

From: Marc Zyngier <maz@kernel.org>
To: Sebastian Ott <sebott@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Oliver Upton <oliver.upton@linux.dev>
Subject: Re: [PATCH 3/4] KVM: arm64: show writable masks for feature registers
Date: Mon, 18 Mar 2024 12:03:04 +0000	[thread overview]
Message-ID: <86zfuvzr0n.wl-maz@kernel.org> (raw)
In-Reply-To: <20240318111636.10613-4-sebott@redhat.com>

On Mon, 18 Mar 2024 11:16:35 +0000,
Sebastian Ott <sebott@redhat.com> wrote:
> 
> Instead of using ~0UL provide the actual writable mask for
> non-id feature registers in the output of the
> KVM_ARM_GET_REG_WRITABLE_MASKS ioctl.

CTR_EL0 definitely seems to be in the feature ID range. And so does
CLIDR_EL1.

> 
> This changes the mask for the CTR_EL0 register.

Only that?

> 
> Signed-off-by: Sebastian Ott <sebott@redhat.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 21 ++++++---------------
>  1 file changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index b2019faa9d73..0f8fe7790c35 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2489,12 +2489,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  
>  	{ SYS_DESC(SYS_CCSIDR_EL1), access_ccsidr },
>  	{ SYS_DESC(SYS_CLIDR_EL1), access_clidr, reset_clidr, CLIDR_EL1,
> -	  .set_user = set_clidr },
> +	  .set_user = set_clidr, .val = ~0UL },

How is CLIDR_EL1 (and all the crap that depend on it) recomputed when
CTR_EL0 is changed?

>  	{ SYS_DESC(SYS_CCSIDR2_EL1), undef_access },
>  	{ SYS_DESC(SYS_SMIDR_EL1), undef_access },
>  	{ SYS_DESC(SYS_CSSELR_EL1), access_csselr, reset_unknown, CSSELR_EL1 },
>  	{ SYS_DESC(SYS_CTR_EL0), access_ctr, .reset = reset_ctr,
> -	  .get_user = get_ctr, .set_user = set_ctr},
> +	  .get_user = get_ctr, .set_user = set_ctr, .val = CTR_EL0_DIC_MASK},

This is all extremely fragile. We need a better solution for this. And
we need far more than just DIC here.

>  	{ SYS_DESC(SYS_SVCR), undef_access },
>  
>  	{ PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
> @@ -3934,20 +3934,11 @@ int kvm_vm_ioctl_get_reg_writable_masks(struct kvm *kvm, struct reg_mask_range *
>  		if (!is_feature_id_reg(encoding) || !reg->set_user)
>  			continue;
>  
> -		/*
> -		 * For ID registers, we return the writable mask. Other feature
> -		 * registers return a full 64bit mask. That's not necessary
> -		 * compliant with a given revision of the architecture, but the
> -		 * RES0/RES1 definitions allow us to do that.
> -		 */
> -		if (is_id_reg(encoding)) {
> -			if (!reg->val ||
> -			    (is_aa32_id_reg(encoding) && !kvm_supports_32bit_el0()))
> -				continue;
> -			val = reg->val;
> -		} else {
> -			val = ~0UL;
> +		if (!reg->val ||
> +		    (is_aa32_id_reg(encoding) && !kvm_supports_32bit_el0())) {
> +			continue;
>  		}
> +		val = reg->val;

Are CLIDR_EL1 and CTR_EL0 the only two ID registers that had a
set_user callback without an encoded mask?

	M.

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

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