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Date: Tue, 20 Feb 2024 15:27:22 +0100
From: Lorenzo Pieralisi <lpieralisi@kernel.org>
To: ankita@nvidia.com
Cc: jgg@nvidia.com, maz@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH v8 1/4] kvm: arm64: introduce new flag for non-cacheable
 IO memory
Message-ID: <ZdS2yl1kzZv/h+vH@lpieralisi>
References: <20240220072926.6466-1-ankita@nvidia.com>
 <20240220072926.6466-2-ankita@nvidia.com>
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On Tue, Feb 20, 2024 at 12:59:23PM +0530, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> Currently, KVM for ARM64 maps at stage 2 memory that is considered device
> (i.e. it is not RAM) with DEVICE_nGnRE memory attributes; this setting
> overrides (as per the ARM architecture [1]) any device MMIO mapping
> present at stage 1, resulting in a set-up whereby a guest operating
> system cannot determine device MMIO mapping memory attributes on its
> own but it is always overridden by the KVM stage 2 default.
> 
> This set-up does not allow guest operating systems to select device
> memory attributes independently from KVM stage-2 mappings
> (refer to [1], "Combining stage 1 and stage 2 memory type attributes"),
> which turns out to be an issue in that guest operating systems
> (e.g. Linux) may request to map devices MMIO regions with memory
> attributes that guarantee better performance (e.g. gathering
> attribute - that for some devices can generate larger PCIe memory
> writes TLPs) and specific operations (e.g. unaligned transactions)
> such as the NormalNC memory type.
> 
> The default device stage 2 mapping was chosen in KVM for ARM64 since
> it was considered safer (i.e. it would not allow guests to trigger
> uncontained failures ultimately crashing the machine) but this
> turned out to be asynchronous (SError) defeating the purpose.
> 
> Failures containability is a property of the platform and is independent
> from the memory type used for MMIO device memory mappings.
> 
> Actually, DEVICE_nGnRE memory type is even more problematic than
> Normal-NC memory type in terms of faults containability in that e.g.
> aborts triggered on DEVICE_nGnRE loads cannot be made, architecturally,
> synchronous (i.e. that would imply that the processor should issue at
> most 1 load transaction at a time - it cannot pipeline them - otherwise
> the synchronous abort semantics would break the no-speculation attribute
> attached to DEVICE_XXX memory).
> 
> This means that regardless of the combined stage1+stage2 mappings a
> platform is safe if and only if device transactions cannot trigger
> uncontained failures and that in turn relies on platform capabilities
> and the device type being assigned (i.e. PCIe AER/DPC error containment
> and RAS architecture[3]); therefore the default KVM device stage 2
> memory attributes play no role in making device assignment safer
> for a given platform (if the platform design adheres to design
> guidelines outlined in [3]) and therefore can be relaxed.
> 
> For all these reasons, relax the KVM stage 2 device memory attributes
> from DEVICE_nGnRE to Normal-NC.
> 
> The NormalNC was chosen over a different Normal memory type default
> at stage-2 (e.g. Normal Write-through) to avoid cache allocation/snooping.
> 
> Relaxing S2 KVM device MMIO mappings to Normal-NC is not expected to
> trigger any issue on guest device reclaim use cases either (i.e. device
> MMIO unmap followed by a device reset) at least for PCIe devices, in that
> in PCIe a device reset is architected and carried out through PCI config
> space transactions that are naturally ordered with respect to MMIO
> transactions according to the PCI ordering rules.
> 
> Having Normal-NC S2 default puts guests in control (thanks to
> stage1+stage2 combined memory attributes rules [1]) of device MMIO
> regions memory mappings, according to the rules described in [1]
> and summarized here ([(S1) - stage1], [(S2) - stage 2]):
> 
> S1           |  S2           | Result
> NORMAL-WB    |  NORMAL-NC    | NORMAL-NC
> NORMAL-WT    |  NORMAL-NC    | NORMAL-NC
> NORMAL-NC    |  NORMAL-NC    | NORMAL-NC
> DEVICE<attr> |  NORMAL-NC    | DEVICE<attr>
> 
> It is worth noting that currently, to map devices MMIO space to user
> space in a device pass-through use case the VFIO framework applies memory
> attributes derived from pgprot_noncached() settings applied to VMAs, which
> result in device-nGnRnE memory attributes for the stage-1 VMM mappings.
> 
> This means that a userspace mapping for device MMIO space carried
> out with the current VFIO framework and a guest OS mapping for the same
> MMIO space may result in a mismatched alias as described in [2].
> 
> Defaulting KVM device stage-2 mappings to Normal-NC attributes does not
> change anything in this respect, in that the mismatched aliases would
> only affect (refer to [2] for a detailed explanation) ordering between
> the userspace and GuestOS mappings resulting stream of transactions
> (i.e. it does not cause loss of property for either stream of
> transactions on its own), which is harmless given that the userspace
> and GuestOS access to the device is carried out through independent
> transactions streams.
> 
> 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 - DDI0487J_a_a-profile_architecture_reference_manual.pdf
> [2] section B2.8 - DDI0487J_a_a-profile_architecture_reference_manual.pdf
> [3] sections 1.7.7.3/1.8.5.2/appendix C - DEN0029H_SBSA_7.1.pdf
> 

A couple of Link: tags would not hurt in my opinion, lest we forget:

Link: https://lore.kernel.org/r/20230907181459.18145-2-ankita@nvidia.com/
Link: https://lore.kernel.org/r/20231205033015.10044-1-ankita@nvidia.com

> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-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         | 24 +++++++++++++++++++-----
>  3 files changed, 23 insertions(+), 5 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 d82305ab420f..449ca2ff1df6 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -173,6 +173,7 @@
>   * Memory types for Stage-2 translation
>   */
>  #define MT_S2_NORMAL		0xf
> +#define MT_S2_NORMAL_NC		0x5
>  #define MT_S2_DEVICE_nGnRE	0x1
>  
>  /*
> @@ -180,6 +181,7 @@
>   * 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 ab9d05fcf98b..3fae5830f8d2 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -717,15 +717,29 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
>  static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot prot,
>  				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);
> +	kvm_pte_t attr;
>  	u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
>  
> +	switch (prot & (KVM_PGTABLE_PROT_DEVICE |
> +			KVM_PGTABLE_PROT_NORMAL_NC)) {
> +	case KVM_PGTABLE_PROT_DEVICE | KVM_PGTABLE_PROT_NORMAL_NC:
> +		return -EINVAL;
> +	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:
> +		if (prot & KVM_PGTABLE_PROT_X)
> +			return -EINVAL;
> +		attr = KVM_S2_MEMATTR(pgt, NORMAL_NC);
> +		break;
> +	default:
> +		attr = KVM_S2_MEMATTR(pgt, NORMAL);
> +	}
> +
>  	if (!(prot & KVM_PGTABLE_PROT_X))
>  		attr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
> -	else if (device)
> -		return -EINVAL;
>  
>  	if (prot & KVM_PGTABLE_PROT_R)
>  		attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R;
> -- 
> 2.34.1
>