Re: [PATCH v3 1/2] kvm: arm64: introduce new flag for non-cacheable IO memory

[Will Deacon <will@kernel.org>]

On Fri, Dec 08, 2023 at 10:17:08PM +0530, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> For various reasons described in the cover letter, and primarily to
> 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@nvidia.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> Tested-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  2 ++
>  arch/arm64/include/asm/memory.h      |  2 ++
>  arch/arm64/kvm/hyp/pgtable.c         | 11 +++++++++--
>  3 files changed, 13 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..d4835d553c61 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -718,10 +718,17 @@ 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;
>  
> +	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);

I think it would be worth rejecting the case where both
KVM_PGTABLE_PROT_DEVICE and KVM_PGTABLE_PROT_NORMAL_NC are passed, since
that's clearly a bug in the caller and silently going with device is
arbitrary and confusing.

Will