Re: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

[Chenyi Qiang]

On 12/2/2022 2:13 PM, Chao Peng wrote:
> In confidential computing usages, whether a page is private or shared is
> necessary information for KVM to perform operations like page fault
> handling, page zapping etc. There are other potential use cases for
> per-page memory attributes, e.g. to make memory read-only (or no-exec,
> or exec-only, etc.) without having to modify memslots.
> 
> Introduce two ioctls (advertised by KVM_CAP_MEMORY_ATTRIBUTES) to allow
> userspace to operate on the per-page memory attributes.
>   - KVM_SET_MEMORY_ATTRIBUTES to set the per-page memory attributes to
>     a guest memory range.
>   - KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES to return the KVM supported
>     memory attributes.
> 
> KVM internally uses xarray to store the per-page memory attributes.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@xxxxxxxxxx/
> ---
>  Documentation/virt/kvm/api.rst | 63 ++++++++++++++++++++++++++++
>  arch/x86/kvm/Kconfig           |  1 +
>  include/linux/kvm_host.h       |  3 ++
>  include/uapi/linux/kvm.h       | 17 ++++++++
>  virt/kvm/Kconfig               |  3 ++
>  virt/kvm/kvm_main.c            | 76 ++++++++++++++++++++++++++++++++++
>  6 files changed, 163 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 5617bc4f899f..bb2f709c0900 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5952,6 +5952,59 @@ delivery must be provided via the "reg_aen" struct.
>  The "pad" and "reserved" fields may be used for future extensions and should be
>  set to 0s by userspace.
>  
> +4.138 KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES
> +-----------------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: u64 memory attributes bitmask(out)
> +:Returns: 0 on success, <0 on error
> +
> +Returns supported memory attributes bitmask. Supported memory attributes will
> +have the corresponding bits set in u64 memory attributes bitmask.
> +
> +The following memory attributes are defined::
> +
> +  #define KVM_MEMORY_ATTRIBUTE_READ              (1ULL << 0)
> +  #define KVM_MEMORY_ATTRIBUTE_WRITE             (1ULL << 1)
> +  #define KVM_MEMORY_ATTRIBUTE_EXECUTE           (1ULL << 2)
> +  #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> +
> +4.139 KVM_SET_MEMORY_ATTRIBUTES
> +-----------------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: struct kvm_memory_attributes(in/out)
> +:Returns: 0 on success, <0 on error
> +
> +Sets memory attributes for pages in a guest memory range. Parameters are
> +specified via the following structure::
> +
> +  struct kvm_memory_attributes {
> +	__u64 address;
> +	__u64 size;
> +	__u64 attributes;
> +	__u64 flags;
> +  };
> +
> +The user sets the per-page memory attributes to a guest memory range indicated
> +by address/size, and in return KVM adjusts address and size to reflect the
> +actual pages of the memory range have been successfully set to the attributes.
> +If the call returns 0, "address" is updated to the last successful address + 1
> +and "size" is updated to the remaining address size that has not been set
> +successfully. The user should check the return value as well as the size to
> +decide if the operation succeeded for the whole range or not. The user may want
> +to retry the operation with the returned address/size if the previous range was
> +partially successful.
> +
> +Both address and size should be page aligned and the supported attributes can be
> +retrieved with KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES.
> +
> +The "flags" field may be used for future extensions and should be set to 0s.
> +
>  5. The kvm_run structure
>  ========================
>  
> @@ -8270,6 +8323,16 @@ structure.
>  When getting the Modified Change Topology Report value, the attr->addr
>  must point to a byte where the value will be stored or retrieved from.
>  
> +8.40 KVM_CAP_MEMORY_ATTRIBUTES
> +------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm
> +
> +This capability indicates KVM supports per-page memory attributes and ioctls
> +KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES/KVM_SET_MEMORY_ATTRIBUTES are available.
> +
>  9. Known KVM API problems
>  =========================
>  
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index fbeaa9ddef59..a8e379a3afee 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -49,6 +49,7 @@ config KVM
>  	select SRCU
>  	select INTERVAL_TREE
>  	select HAVE_KVM_PM_NOTIFIER if PM
> +	select HAVE_KVM_MEMORY_ATTRIBUTES
>  	help
>  	  Support hosting fully virtualized guest machines using hardware
>  	  virtualization extensions.  You will need a fairly recent
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 8f874a964313..a784e2b06625 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -800,6 +800,9 @@ struct kvm {
>  
>  #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
>  	struct notifier_block pm_notifier;
> +#endif
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +	struct xarray mem_attr_array;
>  #endif
>  	char stats_id[KVM_STATS_NAME_SIZE];
>  };
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 64dfe9c07c87..5d0941acb5bb 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1182,6 +1182,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_S390_CPU_TOPOLOGY 222
>  #define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
>  #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
> +#define KVM_CAP_MEMORY_ATTRIBUTES 225
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> @@ -2238,4 +2239,20 @@ struct kvm_s390_zpci_op {
>  /* flags for kvm_s390_zpci_op->u.reg_aen.flags */
>  #define KVM_S390_ZPCIOP_REGAEN_HOST    (1 << 0)
>  
> +/* Available with KVM_CAP_MEMORY_ATTRIBUTES */
> +#define KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES    _IOR(KVMIO,  0xd2, __u64)
> +#define KVM_SET_MEMORY_ATTRIBUTES              _IOWR(KVMIO,  0xd3, struct kvm_memory_attributes)
> +
> +struct kvm_memory_attributes {
> +	__u64 address;
> +	__u64 size;
> +	__u64 attributes;
> +	__u64 flags;
> +};
> +
> +#define KVM_MEMORY_ATTRIBUTE_READ              (1ULL << 0)
> +#define KVM_MEMORY_ATTRIBUTE_WRITE             (1ULL << 1)
> +#define KVM_MEMORY_ATTRIBUTE_EXECUTE           (1ULL << 2)
> +#define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> +
>  #endif /* __LINUX_KVM_H */
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 800f9470e36b..effdea5dd4f0 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -19,6 +19,9 @@ config HAVE_KVM_IRQ_ROUTING
>  config HAVE_KVM_DIRTY_RING
>         bool
>  
> +config HAVE_KVM_MEMORY_ATTRIBUTES
> +       bool
> +
>  # Only strongly ordered architectures can select this, as it doesn't
>  # put any explicit constraint on userspace ordering. They can also
>  # select the _ACQ_REL version.
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 1782c4555d94..7f0f5e9f2406 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1150,6 +1150,9 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
>  	spin_lock_init(&kvm->mn_invalidate_lock);
>  	rcuwait_init(&kvm->mn_memslots_update_rcuwait);
>  	xa_init(&kvm->vcpu_array);
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +	xa_init(&kvm->mem_attr_array);
> +#endif
>  
>  	INIT_LIST_HEAD(&kvm->gpc_list);
>  	spin_lock_init(&kvm->gpc_lock);
> @@ -1323,6 +1326,9 @@ static void kvm_destroy_vm(struct kvm *kvm)
>  		kvm_free_memslots(kvm, &kvm->__memslots[i][0]);
>  		kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
>  	}
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +	xa_destroy(&kvm->mem_attr_array);
> +#endif
>  	cleanup_srcu_struct(&kvm->irq_srcu);
>  	cleanup_srcu_struct(&kvm->srcu);
>  	kvm_arch_free_vm(kvm);
> @@ -2323,6 +2329,49 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
>  }
>  #endif /* CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT */
>  
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +static u64 kvm_supported_mem_attributes(struct kvm *kvm)
> +{
> +	return 0;
> +}
> +
> +static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> +					   struct kvm_memory_attributes *attrs)
> +{
> +	gfn_t start, end;
> +	unsigned long i;
> +	void *entry;
> +	u64 supported_attrs = kvm_supported_mem_attributes(kvm);
> +
> +	/* flags is currently not used. */
> +	if (attrs->flags)
> +		return -EINVAL;
> +	if (attrs->attributes & ~supported_attrs)
> +		return -EINVAL;
> +	if (attrs->size == 0 || attrs->address + attrs->size < attrs->address)
> +		return -EINVAL;
> +	if (!PAGE_ALIGNED(attrs->address) || !PAGE_ALIGNED(attrs->size))
> +		return -EINVAL;
> +
> +	start = attrs->address >> PAGE_SHIFT;
> +	end = (attrs->address + attrs->size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
> +
> +	entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
> +

Because guest memory defaults to private, and now this patch stores the
attributes with KVM_MEMORY_ATTRIBUTE_PRIVATE instead of _SHARED, it
would bring more KVM_EXIT_MEMORY_FAULT exits at the beginning of boot
time. Maybe it can be optimized somehow in other places? e.g. set mem
attr in advance.

> +	mutex_lock(&kvm->lock);
> +	for (i = start; i < end; i++)
> +		if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> +				    GFP_KERNEL_ACCOUNT)))
> +			break;
> +	mutex_unlock(&kvm->lock);
> +
> +	attrs->address = i << PAGE_SHIFT;
> +	attrs->size = (end - i) << PAGE_SHIFT;
> +
> +	return 0;
> +}
> +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
> +
>  struct kvm_memory_slot *gfn_to_memslot(struct kvm *kvm, gfn_t gfn)
>  {
>  	return __gfn_to_memslot(kvm_memslots(kvm), gfn);