Date:   Mon, 19 Jul 2021 15:24:32 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
        tabba@...gle.com, dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH 07/14] KVM: arm64: Enable forcing page-level stage-2 mappings

On Mon, 19 Jul 2021 11:47:28 +0100,
Quentin Perret <qperret@...gle.com> wrote:
> 
> Much of the stage-2 manipulation logic relies on being able to destroy
> block mappings if e.g. installing a smaller mapping in the range. The
> rationale for this behaviour is that stage-2 mappings can always be
> re-created lazily. However, this gets more complicated when the stage-2
> page-table is used to store metadata about the underlying pages. In such
> a case, destroying a block mapping may lead to losing part of the
> state, and confuse the user of those metadata (such as the hypervisor in
> nVHE protected mode).
> 
> To fix this, introduce a callback function in the pgtable struct which
> is called during all map operations to determine whether the mappings
> can us blocks, or should be forced to page-granularity level. This is

nit: use?

> used by the hypervisor when creating the host stage-2 to force
> page-level mappings when using non-default protection attributes.
> 
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h  | 63 +++++++++++++++++----------
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 16 +++++--
>  arch/arm64/kvm/hyp/pgtable.c          | 20 +++++++--
>  3 files changed, 69 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index af62203d2f7a..dd72653314c7 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -75,25 +75,6 @@ enum kvm_pgtable_stage2_flags {
>  	KVM_PGTABLE_S2_IDMAP			= BIT(1),
>  };
>  
> -/**
> - * struct kvm_pgtable - KVM page-table.
> - * @ia_bits:		Maximum input address size, in bits.
> - * @start_level:	Level at which the page-table walk starts.
> - * @pgd:		Pointer to the first top-level entry of the page-table.
> - * @mm_ops:		Memory management callbacks.
> - * @mmu:		Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
> - */
> -struct kvm_pgtable {
> -	u32					ia_bits;
> -	u32					start_level;
> -	kvm_pte_t				*pgd;
> -	struct kvm_pgtable_mm_ops		*mm_ops;
> -
> -	/* Stage-2 only */
> -	struct kvm_s2_mmu			*mmu;
> -	enum kvm_pgtable_stage2_flags		flags;
> -};
> -
>  /**
>   * enum kvm_pgtable_prot - Page-table permissions and attributes.
>   * @KVM_PGTABLE_PROT_X:		Execute permission.
> @@ -109,11 +90,41 @@ enum kvm_pgtable_prot {
>  	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
>  };
>  
> -#define PAGE_HYP		(KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> +#define KVM_PGTABLE_PROT_RW	(KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> +#define KVM_PGTABLE_PROT_RWX	(KVM_PGTABLE_PROT_RW | KVM_PGTABLE_PROT_X)
> +
> +#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)
>  
> +typedef bool (*kvm_pgtable_want_pte_cb_t)(u64 addr, u64 end,
> +					  enum kvm_pgtable_prot prot);
> +
> +/**
> + * struct kvm_pgtable - KVM page-table.
> + * @ia_bits:		Maximum input address size, in bits.
> + * @start_level:	Level at which the page-table walk starts.
> + * @pgd:		Pointer to the first top-level entry of the page-table.
> + * @mm_ops:		Memory management callbacks.
> + * @mmu:		Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
> + * @flags:		Stage-2 page-table flags.
> + * @want_pte_cb:	Callback function used during map operations to decide
> + *			whether block mappings can be used to map the given IPA
> + *			range.
> + */
> +struct kvm_pgtable {
> +	u32					ia_bits;
> +	u32					start_level;
> +	kvm_pte_t				*pgd;
> +	struct kvm_pgtable_mm_ops		*mm_ops;
> +
> +	/* Stage-2 only */
> +	struct kvm_s2_mmu			*mmu;
> +	enum kvm_pgtable_stage2_flags		flags;
> +	kvm_pgtable_want_pte_cb_t		want_pte_cb;
> +};

nit: does this whole definition really need to move around?

> +
>  /**
>   * struct kvm_mem_range - Range of Intermediate Physical Addresses
>   * @start:	Start of the range.
> @@ -216,21 +227,25 @@ int kvm_pgtable_hyp_map(struct kvm_pgtable *pgt, u64 addr, u64 size, u64 phys,
>  u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift);
>  
>  /**
> - * kvm_pgtable_stage2_init_flags() - Initialise a guest stage-2 page-table.
> + * kvm_pgtable_stage2_init_full() - Initialise a guest stage-2 page-table.
>   * @pgt:	Uninitialised page-table structure to initialise.
>   * @arch:	Arch-specific KVM structure representing the guest virtual
>   *		machine.
>   * @mm_ops:	Memory management callbacks.
>   * @flags:	Stage-2 configuration flags.
> + * @want_pte_cb: Callback function used during map operations to decide
> + *		whether block mappings can be used to map the given IPA
> + *		range.
>   *
>   * Return: 0 on success, negative error code on failure.
>   */
> -int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> +int kvm_pgtable_stage2_init_full(struct kvm_pgtable *pgt, struct kvm_arch *arch,
>  				  struct kvm_pgtable_mm_ops *mm_ops,
> -				  enum kvm_pgtable_stage2_flags flags);
> +				  enum kvm_pgtable_stage2_flags flags,
> +				  kvm_pgtable_want_pte_cb_t want_pte_cb);
>  
>  #define kvm_pgtable_stage2_init(pgt, arch, mm_ops) \
> -	kvm_pgtable_stage2_init_flags(pgt, arch, mm_ops, 0)
> +	kvm_pgtable_stage2_init_full(pgt, arch, mm_ops, 0, NULL)

nit: in general, we use __foo() as the primitive for foo(), rather
than foo_with_icing_on_top().

>  
>  /**
>   * kvm_pgtable_stage2_destroy() - Destroy an unused guest stage-2 page-table.
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 58edc62be6f7..cdace80d3e28 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -89,6 +89,7 @@ static void prepare_host_vtcr(void)
>  					  id_aa64mmfr1_el1_sys_val, phys_shift);
>  }
>  
> +static bool host_stage2_want_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot prot);
>  int kvm_host_prepare_stage2(void *pgt_pool_base)
>  {
>  	struct kvm_s2_mmu *mmu = &host_kvm.arch.mmu;
> @@ -101,8 +102,9 @@ int kvm_host_prepare_stage2(void *pgt_pool_base)
>  	if (ret)
>  		return ret;
>  
> -	ret = kvm_pgtable_stage2_init_flags(&host_kvm.pgt, &host_kvm.arch,
> -					    &host_kvm.mm_ops, KVM_HOST_S2_FLAGS);
> +	ret = kvm_pgtable_stage2_init_full(&host_kvm.pgt, &host_kvm.arch,
> +					   &host_kvm.mm_ops, KVM_HOST_S2_FLAGS,
> +					   host_stage2_want_pte_cb);
>  	if (ret)
>  		return ret;
>  
> @@ -225,9 +227,17 @@ static inline int __host_stage2_idmap(u64 start, u64 end,
>  		__ret;							\
>  	 })
>  
> +static bool host_stage2_want_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot prot)
> +{
> +	if (range_is_memory(addr, end))
> +		return prot != KVM_PGTABLE_PROT_RWX;
> +	else
> +		return prot != KVM_PGTABLE_PROT_RW;
> +}

This really deserves a comment about *why* you make such decision. I
also find it a bit odd that you use the permissions to decide whether
to map a block or a not. It feels like the permission is more of a
side effect than anything else.

> +
>  static int host_stage2_idmap(u64 addr)
>  {
> -	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W;
> +	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_RW;
>  	struct kvm_mem_range range;
>  	bool is_memory = find_mem_range(addr, &range);
>  	int ret;
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 34cf67997a82..5bdbe7a31551 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -452,6 +452,8 @@ int kvm_pgtable_hyp_init(struct kvm_pgtable *pgt, u32 va_bits,
>  	pgt->start_level	= KVM_PGTABLE_MAX_LEVELS - levels;
>  	pgt->mm_ops		= mm_ops;
>  	pgt->mmu		= NULL;
> +	pgt->want_pte_cb	= NULL;
> +
>  	return 0;
>  }
>  
> @@ -491,6 +493,7 @@ struct stage2_map_data {
>  	struct kvm_pgtable_mm_ops	*mm_ops;
>  
>  	int				ret;
> +	bool				force_pte;

OK, so you have *two* mechanisms here: once to decide if a range can
be mapped as a block or not, and another one to remember the result
while walking the S2 PTW. This probably deserves some documentation
and/or patch splitting.

>  };
>  
>  u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift)
> @@ -613,6 +616,9 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  	struct kvm_pgtable *pgt = data->mmu->pgt;
>  	struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
>  
> +	if (data->force_pte && (level < (KVM_PGTABLE_MAX_LEVELS - 1)))
> +		return -E2BIG;
> +
>  	if (!kvm_block_mapping_supported(addr, end, phys, level))
>  		return -E2BIG;
>  
> @@ -660,6 +666,9 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
>  	if (data->anchor)
>  		return 0;
>  
> +	if (data->force_pte && (level < (KVM_PGTABLE_MAX_LEVELS - 1)))
> +		return 0;
> +
>  	if (!kvm_block_mapping_supported(addr, end, data->phys, level))

There is something in me screaming that kvm_block_mapping_supported()
should be the point where we check for these things...  Or at least a
helper function that takes 'data' as a parameter.

>  		return 0;
>  
> @@ -791,6 +800,7 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  		.memcache	= mc,
>  		.mm_ops		= pgt->mm_ops,
>  		.ret		= 0,
> +		.force_pte	= pgt->want_pte_cb && pgt->want_pte_cb(addr, addr + size, prot),

Reading this makes me want to rename want_pte_cb() to force_pte_cb()...

>  	};
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_map_walker,
> @@ -826,6 +836,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  		.mm_ops		= pgt->mm_ops,
>  		.owner_id	= owner_id,
>  		.ret		= 0,
> +		.force_pte	= true,
>  	};
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_map_walker,
> @@ -1070,9 +1081,11 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
>  	return kvm_pgtable_walk(pgt, addr, size, &walker);
>  }
>  
> -int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> -				  struct kvm_pgtable_mm_ops *mm_ops,
> -				  enum kvm_pgtable_stage2_flags flags)
> +
> +int kvm_pgtable_stage2_init_full(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> +				 struct kvm_pgtable_mm_ops *mm_ops,
> +				 enum kvm_pgtable_stage2_flags flags,
> +				 kvm_pgtable_want_pte_cb_t want_pte_cb)
>  {
>  	size_t pgd_sz;
>  	u64 vtcr = arch->vtcr;
> @@ -1090,6 +1103,7 @@ int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch
>  	pgt->mm_ops		= mm_ops;
>  	pgt->mmu		= &arch->mmu;
>  	pgt->flags		= flags;
> +	pgt->want_pte_cb	= want_pte_cb;
>  
>  	/* Ensure zeroed PGD pages are visible to the hardware walker */
>  	dsb(ishst);

Thanks,

	M.

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

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