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SubjectRe: [PATCH v10 2/9] KVM: Introduce per-page memory attributes
Date
Chao Peng <chao.p.peng@linux.intel.com> 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@google.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@google.com/
> ---
> 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;

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