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

From: Gupta, Pankaj
Date: Thu Aug 11 2022 - 19:03:16 EST

On 8/11/2022 7:18 PM, Nikunj A. Dadhania wrote:
On 11/08/22 17:00, Gupta, Pankaj 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.

mm extension
---------------------
Introduces new MFD_INACCESSIBLE flag for memfd_create(), the file
created with these flags cannot read(), write() or mmap() etc via normal
MMU operations. The file content can only be used with the newly
introduced memfile_notifier extension.

The memfile_notifier extension provides two sets of callbacks for KVM to
interact with the memory backing store:
   - memfile_notifier_ops: callbacks for memory backing store to notify
     KVM when memory gets invalidated.
   - backing store callbacks: callbacks for KVM to call into memory
     backing store to request memory pages for guest private memory.

The memfile_notifier extension also provides APIs for memory backing
store to register/unregister itself and to trigger the notifier when the
bookmarked memory gets invalidated.

The patchset also introduces a new memfd seal F_SEAL_AUTO_ALLOCATE to
prevent double allocation caused by unintentional guest when we only
have a single side of the shared/private memfds effective.

memslot extension
-----------------
Add the private fd and the fd offset to existing 'shared' memslot so
that both private/shared guest memory can live in one single memslot.
A page in the memslot is either private or shared. Whether a guest page
is private or shared is maintained through reusing existing SEV ioctls
KVM_MEMORY_ENCRYPT_{UN,}REG_REGION.

Test
----
To test the new functionalities of this patch TDX patchset is needed.
Since TDX patchset has not been merged so I did two kinds of test:

-  Regresion test on kvm/queue (this patchset)
    Most new code are not covered. Code also in below repo:
    https://github.com/chao-p/linux/tree/privmem-v7

-  New Funational test on latest TDX code
    The patch is rebased to latest TDX code and tested the new
    funcationalities. See below repos:
    Linux: https://github.com/chao-p/linux/tree/privmem-v7-tdx
    QEMU: https://github.com/chao-p/qemu/tree/privmem-v7

While debugging an issue with SEV+UPM, found that fallocate() returns
an error in QEMU which is not handled (EINTR). With the below handling
of EINTR subsequent fallocate() succeeds:


diff --git a/backends/hostmem-memfd-private.c b/backends/hostmem-memfd-private.c
index af8fb0c957..e8597ed28d 100644
--- a/backends/hostmem-memfd-private.c
+++ b/backends/hostmem-memfd-private.c
@@ -39,7 +39,7 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
      MachineState *machine = MACHINE(qdev_get_machine());
      uint32_t ram_flags;
      char *name;
-    int fd, priv_fd;
+    int fd, priv_fd, ret;
        if (!backend->size) {
          error_setg(errp, "can't create backend with size 0");
@@ -65,7 +65,15 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                     backend->size, ram_flags, fd, 0, errp);
      g_free(name);
  -    fallocate(priv_fd, 0, 0, backend->size);
+again:
+    ret = fallocate(priv_fd, 0, 0, backend->size);
+    if (ret) {
+           perror("Fallocate failed: \n");
+           if (errno == EINTR)
+                   goto again;
+           else
+                   exit(1);
+    }

However, fallocate() preallocates full guest memory before starting the guest.
With this behaviour guest memory is *not* demand pinned. Is there a way to
prevent fallocate() from reserving full guest memory?

Isn't the pinning being handled by the corresponding host memory backend with mmu > notifier and architecture support while doing the memory operations e.g page> migration and swapping/reclaim (not supported currently AFAIU). But yes, we need> to allocate entire guest memory with the new flags MEMFILE_F_{UNMOVABLE, UNRECLAIMABLE etc}.

That is correct, but the question is when does the memory allocated, as these flags are set,
memory is neither moved nor reclaimed. In current scenario, if I start a 32GB guest, all 32GB is
allocated.

I guess so if guest memory is private by default.

Other option would be to allocate memory as shared by default and
handle on demand allocation and RMPUPDATE with page state change event. But still that would be done at guest boot time, IIUC.

Might be missing some details on this. So, better to wait for someone more familiar to answer.

Thanks,
Pankaj