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

[Chao Peng]

On Tue, Dec 06, 2022 at 10:34:32AM -0300, Fabiano Rosas wrote:
> 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"

Thanks, this is much better.

> 
> > +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?

It indeed becomes better if the size is number of pages and the address
is gfn, but I think we don't want to imply that the page size is 4K to
userspace.

> 
> > 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.

Correct!

> 
> Consider using gpa_to_gfn as well.

Yes using gpa_to_gfn is appropriate.

Thanks,
Chao
> 
> > +
> > +	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;