linux-kernel - Re: [PATCH v4 1/3] kvm: arm64: introduce new flag for non-cacheable IO memory

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Message-ID: <87wmt9t137.wl-maz@kernel.org>
Date: Wed, 20 Dec 2023 14:12:12 +0000
From: Marc Zyngier <maz@...nel.org>
To: <ankita@...dia.com>
Cc: <jgg@...dia.com>,
	<oliver.upton@...ux.dev>,
	<suzuki.poulose@....com>,
	<yuzenghui@...wei.com>,
	<catalin.marinas@....com>,
	<will@...nel.org>,
	<alex.williamson@...hat.com>,
	<kevin.tian@...el.com>,
	<yi.l.liu@...el.com>,
	<ardb@...nel.org>,
	<akpm@...ux-foundation.org>,
	<gshan@...hat.com>,
	<mochs@...dia.com>,
	<lpieralisi@...nel.org>,
	<aniketa@...dia.com>,
	<cjia@...dia.com>,
	<kwankhede@...dia.com>,
	<targupta@...dia.com>,
	<vsethi@...dia.com>,
	<acurrid@...dia.com>,
	<apopple@...dia.com>,
	<jhubbard@...dia.com>,
	<danw@...dia.com>,
	<linux-mm@...ck.org>,
	<kvmarm@...ts.linux.dev>,
	<kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 1/3] kvm: arm64: introduce new flag for non-cacheable IO memory

On Mon, 18 Dec 2023 09:07:17 +0000,
<ankita@...dia.com> wrote:
> 
> From: Ankit Agrawal <ankita@...dia.com>
> 
> For various reasons described in the cover letter, and primarily to

Well, the cover letter does not end-up in the git tree, so you must
put some actual information here.

> allow VM get IO memory with NORMALNC properties, it is desired
> to relax the KVM stage 2 device memory attributes from DEVICE_nGnRE
> to NormalNC. So set S2 PTE for IO memory as NORMAL_NC.
> 
> A Normal-NC flag is not present today. So add a new kvm_pgtable_prot
> (KVM_PGTABLE_PROT_NORMAL_NC) flag for it, along with its
> corresponding PTE value 0x5 (0b101) determined from [1].
> 
> Lastly, adapt the stage2 PTE property setter function
> (stage2_set_prot_attr) to handle the NormalNC attribute.
> 
> [1] section D8.5.5 of DDI0487J_a_a-profile_architecture_reference_manual.pdf
> 
> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
> Tested-by: Ankit Agrawal <ankita@...dia.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  2 ++
>  arch/arm64/include/asm/memory.h      |  2 ++
>  arch/arm64/kvm/hyp/pgtable.c         | 13 +++++++++++--
>  3 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index cfdf40f734b1..19278dfe7978 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -197,6 +197,7 @@ enum kvm_pgtable_stage2_flags {
>   * @KVM_PGTABLE_PROT_W:		Write permission.
>   * @KVM_PGTABLE_PROT_R:		Read permission.
>   * @KVM_PGTABLE_PROT_DEVICE:	Device attributes.
> + * @KVM_PGTABLE_PROT_NORMAL_NC:	Normal noncacheable attributes.
>   * @KVM_PGTABLE_PROT_SW0:	Software bit 0.
>   * @KVM_PGTABLE_PROT_SW1:	Software bit 1.
>   * @KVM_PGTABLE_PROT_SW2:	Software bit 2.
> @@ -208,6 +209,7 @@ enum kvm_pgtable_prot {
>  	KVM_PGTABLE_PROT_R			= BIT(2),
>  
>  	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
> +	KVM_PGTABLE_PROT_NORMAL_NC		= BIT(4),
>  
>  	KVM_PGTABLE_PROT_SW0			= BIT(55),
>  	KVM_PGTABLE_PROT_SW1			= BIT(56),
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index fde4186cc387..c247e5f29d5a 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -147,6 +147,7 @@
>   * Memory types for Stage-2 translation
>   */
>  #define MT_S2_NORMAL		0xf
> +#define MT_S2_NORMAL_NC		0x5
>  #define MT_S2_DEVICE_nGnRE	0x1
>  
>  /*
> @@ -154,6 +155,7 @@
>   * Stage-2 enforces Normal-WB and Device-nGnRE
>   */
>  #define MT_S2_FWB_NORMAL	6
> +#define MT_S2_FWB_NORMAL_NC	5
>  #define MT_S2_FWB_DEVICE_nGnRE	1
>  
>  #ifdef CONFIG_ARM64_4K_PAGES
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index c651df904fe3..0fff079a0ef3 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -718,10 +718,19 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
>  				kvm_pte_t *ptep)
>  {
>  	bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> -	kvm_pte_t attr = device ? KVM_S2_MEMATTR(pgt, DEVICE_nGnRE) :
> -			    KVM_S2_MEMATTR(pgt, NORMAL);
> +	bool normal_nc = prot & KVM_PGTABLE_PROT_NORMAL_NC;
> +	kvm_pte_t attr;
>  	u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
>  
> +	WARN_ON_ONCE(device && normal_nc);
> +
> +	if (device)
> +		attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRE);
> +	else if (normal_nc)
> +		attr = KVM_S2_MEMATTR(pgt, NORMAL_NC);
> +	else
> +		attr = KVM_S2_MEMATTR(pgt, NORMAL);
> +

This whole thing can be written a bit more elegantly:

	switch (prot & (KVM_PGTABLE_PROT_DEVICE |
			KVM_PGTABLE_PROT_NORMAL_NC)) {
	case 0:
		attr = KVM_S2_MEMATTR(pgt, NORMAL);
		break;
	case KVM_PGTABLE_PROT_DEVICE:
		if (prot & KVM_PGTABLE_PROT_X)
			return -EINVAL;
		attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRE);
		break;
	case KVM_PGTABLE_PROT_NORMAL_NC:
		attr = KVM_S2_MEMATTR(pgt, NORMAL_NC);
		break;
	default:
		WARN_ON_ONCE(1);
	}

and you can get rid of all of the boolean crud.

	M.

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