Re: [RFC PATCH 1/8] KVM: Document KVM_MAP_MEMORY ioctl

[Huang, Kai]

On Fri, 2024-03-01 at 09:28 -0800, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> Adds documentation of KVM_MAP_MEMORY ioctl.
> 
> It pre-populates guest memory. And potentially do initialized memory
> contents with encryption and measurement depending on underlying
> technology.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst | 36 ++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 0b5a33ee71ee..33d2b63f7dbf 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6352,6 +6352,42 @@ a single guest_memfd file, but the bound ranges must not overlap).
>  
>  See KVM_SET_USER_MEMORY_REGION2 for additional details.
>  
> +4.143 KVM_MAP_MEMORY
> +------------------------
> +
> +:Capability: KVM_CAP_MAP_MEMORY
> +:Architectures: none
> +:Type: vcpu ioctl

I think "vcpu ioctl" means theoretically it can be called on multiple vcpus.

What happens in that case?

> +:Parameters: struct kvm_memory_mapping(in/out)
> +:Returns: 0 on success, <0 on error
> +
> +KVM_MAP_MEMORY populates guest memory without running vcpu.
> +
> +::
> +
> +  struct kvm_memory_mapping {
> +	__u64 base_gfn;
> +	__u64 nr_pages;
> +	__u64 flags;
> +	__u64 source;
> +  };
> +
> +  /* For kvm_memory_mapping:: flags */
> +  #define KVM_MEMORY_MAPPING_FLAG_WRITE         _BITULL(0)
> +  #define KVM_MEMORY_MAPPING_FLAG_EXEC          _BITULL(1)
> +  #define KVM_MEMORY_MAPPING_FLAG_USER          _BITULL(2)

I am not sure what's the good of having "FLAG_USER"?

This ioctl is called from userspace, thus I think we can just treat this always
as user-fault?

> +  #define KVM_MEMORY_MAPPING_FLAG_PRIVATE       _BITULL(3)
> +
> +KVM_MAP_MEMORY populates guest memory in the underlying mapping. If source is
> +not zero and it's supported (depending on underlying technology), the guest
> +memory content is populated with the source.  The flags field supports three
> +flags: KVM_MEMORY_MAPPING_FLAG_WRITE, KVM_MEMORY_MAPPING_FLAG_EXEC, and
> +KVM_MEMORY_MAPPING_FLAG_USER.  Which corresponds to fault code for kvm page
> +fault to populate guest memory. write fault, fetch fault and user fault.
> +When it returned, the input is updated.  If nr_pages is large, it may
> +return -EAGAIN and update the values (base_gfn and nr_pages. source if not zero)
> +to point the remaining range.
> +
>  5. The kvm_run structure
>  ========================
>