Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Michael Roth]

On Wed, Jul 06, 2022 at 04:20:02PM +0800, Chao Peng wrote:
> This is the v7 of this series which tries to implement the fd-based KVM
> guest private memory. The patches are based on latest kvm/queue branch
> commit:
> 
>   b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
> split_desc_cache only by default capacity
> 
> Introduction
> ------------
> In general this patch series introduce fd-based memslot which provides
> guest memory through memory file descriptor fd[offset,size] instead of
> hva/size. The fd can be created from a supported memory filesystem
> like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM
> and the the memory backing store exchange callbacks when such memslot
> gets created. At runtime KVM will call into callbacks provided by the
> backing store to get the pfn with the fd+offset. Memory backing store
> will also call into KVM callbacks when userspace punch hole on the fd
> to notify KVM to unmap secondary MMU page table entries.
> 
> Comparing to existing hva-based memslot, this new type of memslot allows
> guest memory unmapped from host userspace like QEMU and even the kernel
> itself, therefore reduce attack surface and prevent bugs.
> 
> Based on this fd-based memslot, we can build guest private memory that
> is going to be used in confidential computing environments such as Intel
> TDX and AMD SEV. When supported, the memory backing store can provide
> more enforcement on the fd and KVM can use a single memslot to hold both
> the private and shared part of the guest memory. 

Hi everyone,

Just wanted to let you all know that I reserved a slot at the LPC
Confidential Computing Microconference to discuss some topics related
to unmapped/inaccessible private memory support:

  "Unmapped Private Memory for Confidential Guests"
  Tuesday, Sep 13th, 10:00am (Dublin time)
  https://lpc.events/event/16/sessions/133/#20220913

The discussion agenda is still a bit in flux, but one topic I really
wanted to cover is how we intend to deal with the kernel directmap
for TDX/SNP, where there is a need to either remove or split mappings
so that KVM or other kernel threads writing to non-private pages
don't run into issues due mappings overlapping with private pages.[1]

Other possible discussion topics:

  - guarding against shared->private conversions while KVM is
    attempting to access a shared page (separate PFN pools for
    shared/private seems to resolve this nicely, but may not be
    compatible with things like pKVM where the underlying PFN
    is the same for shared/private)[2]

  - extending KVM_EXIT_MEMORY_FAULT to handle batched requests to
    better handle things like explicit batched conversions initiated
    by the guest

It's a short session so not sure how much time we'll actually have
to discuss things in detail, but maybe this can at least be a good
jumping off point for other discussions.

Thanks, and hope to see you there!

[1] https://lore.kernel.org/all/YWb8WG6Ravbs1nbx@xxxxxxxxxx/
[2] https://lore.kernel.org/lkml/CA+EHjTy6NF=BkCqK0vhXLdtKZMahp55JUMSfxN96-NT3YiMXYQ@xxxxxxxxxxxxxx/