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

Nicolas Saenz Julienne
2023-05-19T17:33:42Z

Hi,

On Fri Dec 2, 2022 at 6:13 AM UTC, Chao Peng wrote:

[...]

> +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.

We have been looking into adding support for the Hyper-V VSM extensions
which Windows uses to implement Credential Guard. This interface seems
like a good fit for one of its underlying features. I just wanted to
share a bit about it, and see if we can expand it to fit this use-case.
Note that this was already briefly discussed between Sean and Alex some
time ago[1].

VSM introduces isolated guest execution contexts called Virtual Trust
Levels (VTL) [2]. Each VTL has its own memory access protections,
virtual processors states, interrupt controllers and overlay pages. VTLs
are hierarchical and might enforce memory protections on less privileged
VTLs. Memory protections are enforced on a per-GPA granularity.

The list of possible protections is:
- No access -- This needs a new memory attribute, I think.
- Read-only, no execute
- Read-only, execute
- Read/write, no execute
- Read/write, execute

We implemented this in the past by using a separate address space per
VTL and updating memory regions on protection changes. But having to
update the memory slot layout for every permission change scales poorly,
especially as we have to perform 100.000s of these operations at boot
(see [1] for a little more context).

I believe the biggest barrier for us to use memory attributes is not
having the ability to target specific address spaces, or to the very
least having some mechanism to maintain multiple independent layers of
attributes.

Also sorry for not posting our VSM patches. They are not ready for
upstream review yet, but we're working on it.

Nicolas

[1] https://patchwork.kernel.org/comment/25054908/
[2] See Chapter 15 of Microsoft's 'Hypervisor Top Level Functional Specification':
https://raw.githubusercontent.com/MicrosoftDocs/Virtualization-Documentation/main/tlfs/Hypervisor%20Top%20Level%20Functional%20Specification%20v6.0b.pdf