Re: [PATCH v3 1/6] kvm: determine memory type from VMA

From: Marc Zyngier
Date: Wed Apr 12 2023 - 08:44:40 EST

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On Wed, 05 Apr 2023 19:01:29 +0100,
<ankita@xxxxxxxxxx> wrote:
>
> From: Ankit Agrawal <ankita@xxxxxxxxxx>
>
> Each VM stores the requires pgprots for its mappings in the

The VM stores *nothing*. Did you mean VMA? And even then, I can't
parse this.

> vma->pgprot. Based on this we can determine the desired MT_DEVICE_*
> for the VMA directly, and do not have to guess based on heuristics
> based on pfn_is_map_memory().
>
> There are the following kinds of pgprot available to userspace and their
> corresponding type:
> pgprot_noncached -> MT_DEVICE_nGnRnE
> pgprot_writecombine -> MT_NORMAL_NC
> pgprot_device -> MT_DEVICE_nGnRE
> pgprot_tagged -> MT_NORMAL_TAGGED
>
> Decode the relevant MT_* types in use and translate them into the
> corresponding KVM_PGTABLEPROT_*:
>
> - MT_DEVICE_nGnRE -> KVM_PGTABLE_PROT_DEVICE_nGnRE (device)
> - MT_DEVICE_nGnRnE -> KVM_PGTABLE_PROT_DEVICE_nGnRnE (noncached)
> - MT_NORMAL/_TAGGED/_NC -> 0
>
> The selection of 0 for the S2 KVM_PGTABLE_PROT_DEVICE_nGnRnE is based
> on [2].
>
> Also worth noting is the result of the stage-1 and stage-2. Ref [3]
> If FWB not set, then the combination is the one that is more restrictive.
> The sequence from lowest restriction to the highest:
> DEVICE_nGnRnE -> DEVICE_nGnRE -> NORMAL/_TAGGED/_NC

Sorry, but I can't parse this either. If you mean that the strongest
memory type wins, then I agree. But what does it bring to the
discussion?

> If FWB is set, then stage-2 mapping type overrides the stage-1 [1].
>
> This solves a problem where KVM cannot preserve the MT_NORMAL memory
> type for non-struct page backed memory into the S2 mapping. Instead
> the VMA creator determines the MT type and the S2 will follow it.

What makes it safe? How does VFIO ensures that the memory type used is
correct in all circumstances? This has to hold for *ANY* device, not
just your favourite toy of the day. Nothing in this patch is horribly
wrong, but the above question must be answered before we can consider
any of this.

>
> [1] https://developer.arm.com/documentation/102376/0100/Combining-Stage-1-and-Stage-2-attributes
> [2] ARMv8 reference manual: https://developer.arm.com/documentation/ddi0487/gb/ Section D5.5.3, Table D5-38
> [3] ARMv8 reference manual: https://developer.arm.com/documentation/ddi0487/gb/ Table G5-20 on page G5-6330

Please quote references that are current at the time of posting.
Revision DDI0487I.a is the most recent version, so use that.

>
> Signed-off-by: Ankit Agrawal <ankita@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/kvm_pgtable.h | 8 +++++---
> arch/arm64/include/asm/memory.h | 6 ++++--
> arch/arm64/kvm/hyp/pgtable.c | 16 +++++++++++-----
> arch/arm64/kvm/mmu.c | 27 ++++++++++++++++++++++-----
> 4 files changed, 42 insertions(+), 15 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 4cd6762bda80..d3166b6e6329 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -150,7 +150,8 @@ enum kvm_pgtable_stage2_flags {
> * @KVM_PGTABLE_PROT_X: Execute permission.
> * @KVM_PGTABLE_PROT_W: Write permission.
> * @KVM_PGTABLE_PROT_R: Read permission.
> - * @KVM_PGTABLE_PROT_DEVICE: Device attributes.
> + * @KVM_PGTABLE_PROT_DEVICE_nGnRE: Device nGnRE attributes.
> + * @KVM_PGTABLE_PROT_DEVICE_nGnRnE: Device nGnRnE attributes.
> * @KVM_PGTABLE_PROT_SW0: Software bit 0.
> * @KVM_PGTABLE_PROT_SW1: Software bit 1.
> * @KVM_PGTABLE_PROT_SW2: Software bit 2.
> @@ -161,7 +162,8 @@ enum kvm_pgtable_prot {
> KVM_PGTABLE_PROT_W = BIT(1),
> KVM_PGTABLE_PROT_R = BIT(2),
>
> - KVM_PGTABLE_PROT_DEVICE = BIT(3),
> + KVM_PGTABLE_PROT_DEVICE_nGnRE = BIT(3),
> + KVM_PGTABLE_PROT_DEVICE_nGnRnE = BIT(4),
>
> KVM_PGTABLE_PROT_SW0 = BIT(55),
> KVM_PGTABLE_PROT_SW1 = BIT(56),
> @@ -178,7 +180,7 @@ enum kvm_pgtable_prot {
> #define PAGE_HYP KVM_PGTABLE_PROT_RW
> #define PAGE_HYP_EXEC (KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_X)
> #define PAGE_HYP_RO (KVM_PGTABLE_PROT_R)
> -#define PAGE_HYP_DEVICE (PAGE_HYP | KVM_PGTABLE_PROT_DEVICE)
> +#define PAGE_HYP_DEVICE (PAGE_HYP | KVM_PGTABLE_PROT_DEVICE_nGnRE)
>
> typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
> enum kvm_pgtable_prot prot);
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 78e5163836a0..4ebbc4b1ba4d 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -147,14 +147,16 @@
> * Memory types for Stage-2 translation
> */
> #define MT_S2_NORMAL 0xf
> +#define MT_S2_DEVICE_nGnRnE 0x0
> #define MT_S2_DEVICE_nGnRE 0x1
>
> /*
> * Memory types for Stage-2 translation when ID_AA64MMFR2_EL1.FWB is 0001
> * Stage-2 enforces Normal-WB and Device-nGnRE
> */
> -#define MT_S2_FWB_NORMAL 6
> -#define MT_S2_FWB_DEVICE_nGnRE 1
> +#define MT_S2_FWB_NORMAL 0x6
> +#define MT_S2_FWB_DEVICE_nGnRnE 0x0
> +#define MT_S2_FWB_DEVICE_nGnRE 0x1

Why the repainting of perfectly valid constants? What does the 0x
prefix bring? Does the comment above need updating?

>
> #ifdef CONFIG_ARM64_4K_PAGES
> #define IOREMAP_MAX_ORDER (PUD_SHIFT)
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 3d61bd3e591d..7a8238b41590 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -355,7 +355,7 @@ struct hyp_map_data {
>
> static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
> {
> - bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> + bool device = prot & KVM_PGTABLE_PROT_DEVICE_nGnRE;
> u32 mtype = device ? MT_DEVICE_nGnRE : MT_NORMAL;
> kvm_pte_t attr = FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_ATTRIDX, mtype);
> u32 sh = KVM_PTE_LEAF_ATTR_LO_S1_SH_IS;
> @@ -636,14 +636,20 @@ static bool stage2_has_fwb(struct kvm_pgtable *pgt)
> 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);
> u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
> + kvm_pte_t attr;
> +
> + if (prot & KVM_PGTABLE_PROT_DEVICE_nGnRE)
> + attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRE);
> + else if (prot & KVM_PGTABLE_PROT_DEVICE_nGnRnE)
> + attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRnE);
> + else
> + attr = KVM_S2_MEMATTR(pgt, NORMAL);
>
> if (!(prot & KVM_PGTABLE_PROT_X))
> attr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
> - else if (device)
> + else if (prot & KVM_PGTABLE_PROT_DEVICE_nGnRE ||
> + prot & KVM_PGTABLE_PROT_DEVICE_nGnRnE)

Why don't you keep 'device', which is concise and readable, and simply
change the way it is assigned?

> return -EINVAL;
>
> if (prot & KVM_PGTABLE_PROT_R)
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 7113587222ff..8d63aa951c33 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -897,7 +897,7 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> int ret = 0;
> struct kvm_mmu_memory_cache cache = { .gfp_zero = __GFP_ZERO };
> struct kvm_pgtable *pgt = kvm->arch.mmu.pgt;
> - enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_DEVICE |
> + enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_DEVICE_nGnRE |
> KVM_PGTABLE_PROT_R |
> (writable ? KVM_PGTABLE_PROT_W : 0);
>
> @@ -1186,6 +1186,15 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
> return vma->vm_flags & VM_MTE_ALLOWED;
> }
>
> +/*
> + * Determine the memory region cacheability from VMA's pgprot. This
> + * is used to set the stage 2 PTEs.
> + */
> +static unsigned long mapping_type(pgprot_t page_prot)
> +{
> + return ((pgprot_val(page_prot) & PTE_ATTRINDX_MASK) >> 2);

Please use FIELD_GET() for field extraction.

> +}
> +
> static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> struct kvm_memory_slot *memslot, unsigned long hva,
> unsigned long fault_status)
> @@ -1368,10 +1377,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> if (exec_fault)
> prot |= KVM_PGTABLE_PROT_X;
>
> - if (device)
> - prot |= KVM_PGTABLE_PROT_DEVICE;
> - else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
> - prot |= KVM_PGTABLE_PROT_X;
> + switch (mapping_type(vma->vm_page_prot)) {
> + case MT_DEVICE_nGnRE:
> + prot |= KVM_PGTABLE_PROT_DEVICE_nGnRE;
> + break;
> + case MT_DEVICE_nGnRnE:
> + prot |= KVM_PGTABLE_PROT_DEVICE_nGnRnE;
> + break;

Please keep the 'device' guard. It makes it easy to find the code
paths that are relevant to this case.

> + /* MT_NORMAL/_TAGGED/_NC */
> + default:
> + if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
> + prot |= KVM_PGTABLE_PROT_X;
> + }
>
> /*
> * Under the premise of getting a FSC_PERM fault, we just need to relax

Thanks,

M.

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

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