linux-kernel - Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based private memory

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Message-ID: <Y7azFdnnGAdGPqmv@kernel.org>
Date:   Thu, 5 Jan 2023 11:23:01 +0000
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based
 private memory

On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:
> In memory encryption usage, guest memory may be encrypted with special
> key and can be accessed only by the guest itself. We call such memory
> private memory. It's valueless and sometimes can cause problem to allow
> userspace to access guest private memory. This new KVM memslot extension
> allows guest private memory being provided through a restrictedmem
> backed file descriptor(fd) and userspace is restricted to access the
> bookmarked memory in the fd.
> 
> This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> additional KVM memslot fields restricted_fd/restricted_offset to allow
> userspace to instruct KVM to provide guest memory through restricted_fd.
> 'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
> and the size is 'memory_size'.
> 
> The extended memslot can still have the userspace_addr(hva). When use, a
> single memslot can maintain both private memory through restricted_fd
> and shared memory through userspace_addr. Whether the private or shared
> part is visible to guest is maintained by other KVM code.
> 
> A restrictedmem_notifier field is also added to the memslot structure to
> allow the restricted_fd's backing store to notify KVM the memory change,
> KVM then can invalidate its page table entries or handle memory errors.
> 
> Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
> and right now it is selected on X86_64 only.
> 
> To make future maintenance easy, internally use a binary compatible
> alias struct kvm_user_mem_region to handle both the normal and the
> '_ext' variants.

Feels bit hacky IMHO, and more like a completely new feature than
an extension.

Why not just add a new ioctl? The commit message does not address
the most essential design here.

BR, Jarkko