Re: [PATCH RFC gmem v1 3/8] KVM: x86: Add gmem hook for initializing memory

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi,

On 16/10/2023 12:50, Michael Roth wrote:
> guest_memfd pages are generally expected to be in some arch-defined
> initial state prior to using them for guest memory. For SEV-SNP this
> initial state is 'private', or 'guest-owned', and requires additional
> operations to move these pages into a 'private' state by updating the
> corresponding entries the RMP table.
> 
> Allow for an arch-defined hook to handle updates of this sort, and go
> ahead and implement one for x86 so KVM implementations like AMD SVM can
> register a kvm_x86_ops callback to handle these updates for SEV-SNP
> guests.
> 
> The preparation callback is always called when allocating/grabbing
> folios via gmem, and it is up to the architecture to keep track of
> whether or not the pages are already in the expected state (e.g. the RMP
> table in the case of SEV-SNP).
> 
> In some cases, it is necessary to defer the preparation of the pages to
> handle things like in-place encryption of initial guest memory payloads
> before marking these pages as 'private'/'guest-owned', so also add a
> helper that performs the same function as kvm_gmem_get_pfn(), but allows
> for the preparation callback to be bypassed to allow for pages to be
> accessed beforehand.

This will be useful for Arm CCA, where the pages need to be moved into
"Realm state". Some minor comments below.

> 
> Link: https://lore.kernel.org/lkml/ZLqVdvsF11Ddo7Dq@google.com/
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>   arch/x86/include/asm/kvm-x86-ops.h |  1 +
>   arch/x86/include/asm/kvm_host.h    |  2 ++
>   arch/x86/kvm/x86.c                 |  6 ++++
>   include/linux/kvm_host.h           | 14 ++++++++
>   virt/kvm/Kconfig                   |  4 +++
>   virt/kvm/guest_memfd.c             | 56 +++++++++++++++++++++++++++---
>   6 files changed, 78 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index e3054e3e46d5..0c113f42d5c7 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -134,6 +134,7 @@ KVM_X86_OP(msr_filter_changed)
>   KVM_X86_OP(complete_emulated_msr)
>   KVM_X86_OP(vcpu_deliver_sipi_vector)
>   KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> +KVM_X86_OP_OPTIONAL_RET0(gmem_prepare)
>   
>   #undef KVM_X86_OP
>   #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 95018cc653f5..66fc89d1858f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1752,6 +1752,8 @@ struct kvm_x86_ops {
>   	 * Returns vCPU specific APICv inhibit reasons
>   	 */
>   	unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
> +
> +	int (*gmem_prepare)(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
>   };
>   
>   struct kvm_x86_nested_ops {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 767236b4d771..33a4cc33d86d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13301,6 +13301,12 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
>   }
>   EXPORT_SYMBOL_GPL(kvm_arch_no_poll);
>   
> +#ifdef CONFIG_HAVE_KVM_GMEM_PREPARE
> +int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order)
> +{
> +	return static_call(kvm_x86_gmem_prepare)(kvm, pfn, gfn, max_order);
> +}
> +#endif
>   
>   int kvm_spec_ctrl_test_value(u64 value)
>   {
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 8c5c017ab4e9..c7f82c2f1bcf 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2403,9 +2403,19 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
>   
>   #ifdef CONFIG_KVM_PRIVATE_MEM
> +int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prep);
>   int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>   			      gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
>   #else
> +static inline int __kvm_gmem_get_pfn(struct kvm *kvm,
> +				     struct kvm_memory_slot *slot, gfn_t gfn,
> +				     kvm_pfn_t *pfn, int *max_order)

Missing "bool prep" here ?

minor nit: Do we need to export both __kvm_gmem_get_pfn and 
kvm_gmem_get_pfn ? I don't see anyone else using the former.

We could have :

#ifdef CONFIG_KVM_PRIVATE_MEM
int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prep);
#else
static inline int __kvm_gmem_get_pfn(struct kvm *kvm,
				    struct kvm_memory_slot *slot, gfn_t gfn,
				    kvm_pfn_t *pfn, int *max_order,
				    bool prep)
{
	KVM_BUG_ON(1, kvm);
	return -EIO;
}
#endif

static inline int kvm_gmem_get_pfn(struct kvm *kvm,
				 struct kvm_memory_slot *slot, gfn_t gfn,
				kvm_pfn_t *pfn, int *max_order)
{
	return __kvm_gmem_get_pfn(kvm, slot, gfn, pfn, max_order, true);
}


Suzuki

> +	KVM_BUG_ON(1, kvm);
> +	return -EIO;
> +}
> +
>   static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   				   struct kvm_memory_slot *slot, gfn_t gfn,
>   				   kvm_pfn_t *pfn, int *max_order)
> @@ -2415,4 +2425,8 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   }
>   #endif /* CONFIG_KVM_PRIVATE_MEM */
>   
> +#ifdef CONFIG_HAVE_KVM_GMEM_PREPARE
> +int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order);
> +#endif
> +
>   #endif
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 2c964586aa14..992cf6ed86ef 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -109,3 +109,7 @@ config KVM_GENERIC_PRIVATE_MEM
>          select KVM_GENERIC_MEMORY_ATTRIBUTES
>          select KVM_PRIVATE_MEM
>          bool
> +
> +config HAVE_KVM_GMEM_PREPARE
> +       bool
> +       depends on KVM_PRIVATE_MEM
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index f6f1b17a319c..72ff8b7b31d5 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -44,7 +44,40 @@ static struct folio *kvm_gmem_get_huge_folio(struct inode *inode, pgoff_t index)
>   #endif
>   }
>   
> -static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
> +static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct folio *folio)
> +{
> +#ifdef CONFIG_HAVE_KVM_GMEM_PREPARE
> +	struct list_head *gmem_list = &inode->i_mapping->private_list;
> +	struct kvm_gmem *gmem;
> +
> +	list_for_each_entry(gmem, gmem_list, entry) {
> +		struct kvm_memory_slot *slot;
> +		struct kvm *kvm = gmem->kvm;
> +		struct page *page;
> +		kvm_pfn_t pfn;
> +		gfn_t gfn;
> +		int rc;
> +
> +		slot = xa_load(&gmem->bindings, index);
> +		if (!slot)
> +			continue;
> +
> +		page = folio_file_page(folio, index);
> +		pfn = page_to_pfn(page);
> +		gfn = slot->base_gfn + index - slot->gmem.pgoff;
> +		rc = kvm_arch_gmem_prepare(kvm, gfn, pfn, compound_order(compound_head(page)));
> +		if (rc) {
> +			pr_warn_ratelimited("gmem: Failed to prepare folio for index %lx, error %d.\n",
> +					    index, rc);
> +			return rc;
> +		}
> +	}
> +
> +#endif
> +	return 0;
> +}
> +
> +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool prep)
>   {
>   	struct folio *folio;
>   
> @@ -74,6 +107,12 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>   		folio_mark_uptodate(folio);
>   	}
>   
> +	if (prep && kvm_gmem_prepare_folio(inode, index, folio)) {
> +		folio_unlock(folio);
> +		folio_put(folio);
> +		return NULL;
> +	}
> +
>   	/*
>   	 * Ignore accessed, referenced, and dirty flags.  The memory is
>   	 * unevictable and there is no storage to write back to.
> @@ -178,7 +217,7 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
>   			break;
>   		}
>   
> -		folio = kvm_gmem_get_folio(inode, index);
> +		folio = kvm_gmem_get_folio(inode, index, true);
>   		if (!folio) {
>   			r = -ENOMEM;
>   			break;
> @@ -537,8 +576,8 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
>   	fput(file);
>   }
>   
> -int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> -		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
> +int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prep)
>   {
>   	pgoff_t index, huge_index;
>   	struct kvm_gmem *gmem;
> @@ -559,7 +598,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>   		goto out_fput;
>   	}
>   
> -	folio = kvm_gmem_get_folio(file_inode(file), index);
> +	folio = kvm_gmem_get_folio(file_inode(file), index, prep);
>   	if (!folio) {
>   		r = -ENOMEM;
>   		goto out_fput;
> @@ -600,4 +639,11 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>   
>   	return r;
>   }
> +EXPORT_SYMBOL_GPL(__kvm_gmem_get_pfn);
> +
> +int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
> +{
> +	return __kvm_gmem_get_pfn(kvm, slot, gfn, pfn, max_order, true);
> +} >   EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);