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

[Fabiano Rosas]

Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> writes:

> 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.

This wording could cause some confusion, what about a simpler:

"reflect the range of pages that had its attributes successfully set"

> +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.

"address + 1 page" or "subsequent page" perhaps.

In fact, wouldn't this all become simpler if size were number of pages instead?

> 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.
> +

...

> +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;

Here PAGE_SIZE and -1 cancel out.

Consider using gpa_to_gfn as well.

> +
> +	entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
> +
> +	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);
> @@ -4459,6 +4508,9 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>  #ifdef CONFIG_HAVE_KVM_MSI
>  	case KVM_CAP_SIGNAL_MSI:
>  #endif
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +	case KVM_CAP_MEMORY_ATTRIBUTES:
> +#endif
>  #ifdef CONFIG_HAVE_KVM_IRQFD
>  	case KVM_CAP_IRQFD:
>  	case KVM_CAP_IRQFD_RESAMPLE:
> @@ -4804,6 +4856,30 @@ static long kvm_vm_ioctl(struct file *filp,
>  		break;
>  	}
>  #endif /* CONFIG_HAVE_KVM_IRQ_ROUTING */
> +#ifdef CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES
> +	case KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES: {
> +		u64 attrs = kvm_supported_mem_attributes(kvm);
> +
> +		r = -EFAULT;
> +		if (copy_to_user(argp, &attrs, sizeof(attrs)))
> +			goto out;
> +		r = 0;
> +		break;
> +	}
> +	case KVM_SET_MEMORY_ATTRIBUTES: {
> +		struct kvm_memory_attributes attrs;
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&attrs, argp, sizeof(attrs)))
> +			goto out;
> +
> +		r = kvm_vm_ioctl_set_mem_attributes(kvm, &attrs);
> +
> +		if (!r && copy_to_user(argp, &attrs, sizeof(attrs)))
> +			r = -EFAULT;
> +		break;
> +	}
> +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
>  	case KVM_CREATE_DEVICE: {
>  		struct kvm_create_device cd;