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Peter Xu
7ca43d7ad0
Host backends supports guest-memfd now by detecting whether it's a confidential VM. There's no way to choose it yet from the memory level to use it fully shared. If we use guest-memfd, it so far always implies we need two layers of memory backends, while the guest-memfd only provides the private set of pages. This patch introduces a way so that QEMU can consume guest memfd as the only source of memory to back the object (aka, fully shared). To use the fully shared guest-memfd, one can add a memfd object with: -object memory-backend-memfd,x-guest-memfd=on,share=on Note that share=on is required with fully shared guest_memfd. Since guest-memfd is not yet a drop-in replacement for memfd (e.g. lack of hugepage support), the option is tagged as experimental for now so that it can be made available for early enablement/testing without causing any surprises for other users. PS: there's a trivial touch-up on fd<0 check, because the stub to create guest-memfd may return negative but not -1. Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com> Reviewed-by: Fabiano Rosas <farosas@suse.de> Reviewed-by: David Hildenbrand <david@kernel.org> Co-developed-by: Michael Roth <michael.roth@amd.com> Signed-off-by: Michael Roth <michael.roth@amd.com> Acked-by: Markus Armbruster <armbru@redhat.com> Link: https://lore.kernel.org/r/20260929235109.1398509-11-michael.roth@amd.com Signed-off-by: Peter Xu <peterx@redhat.com>
61 lines
2.7 KiB
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61 lines
2.7 KiB
ReStructuredText
.. _guest-memfd:
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guest-memfd support
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===================
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Recent kernels allow for the creation of a guest-memfd file
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descriptor, which can be used to back VMs in a similar manner as a
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memfd file descriptor, but is intended specifically for this purpose
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and allows for closer coordination between KVM and the management of
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this memory to enable more advanced/VM-specific use-cases.
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Initially this additional functionality centered around providing a
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common/centralized place for managing kernel-side memory handing
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requirements for various Confidential Guest architectures. (For more
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on Confidential Guests, see :ref:`confidential-guest-support`).
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guest_memfd has since evolved to become a more general-purpose way to
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allocate/manage guest memory and potentially allow for things like
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providing additional memory isolation within the kernel[1] and support
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for persisting a guest's state across kexec to allow for live-updating
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the host kernel with minimal guest downtime[2].
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Usage
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-----
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For Confidential Guests, guest-memfd is currently utilized internally
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by QEMU to handle private guest memory, independently of whatever
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memory backend the user has configured for normal/non-private/shared
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guest memory. To avoid doubling memory, QEMU discards memory in
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response to the guest converting GPA ranges between shared/private.
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(e.g. if GPA X is converted from private to shared, the guest-memfd FD
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offset corresponding to GPA x will be truncated since the memory will
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be provided by the memory backend the user configured for shared
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memory, and vice-versa). This is handled automatically/internally for
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Confidential Guests that rely on this handling and is not directly
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exposed by QEMU command-line options.
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For non-Confidential guests, guest-memfd can be used in a manner that
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is somewhat interchangeable with a normal memfd. Currently, this is
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handled by using the same memory-backend implementation as memfd, but
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with an additional 'guest-memfd=on' option. E.g.::
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qemu ... \
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-object memory-backend-memfd,id=ID,size=SIZE,share=on,guest-memfd=on
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Note that the share=on option is required for guest-memfd, since it
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does not support anonymous memory allocations or COW-like semantics.
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Also note that there are a couple of limitations in using guest_memfd
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in this way compared to a normal memfd, which is why the option is
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exposed as an experimental for the time being:
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* guest_memfd does not currently support the hugetlb=on option
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* guest_memfd does not currently support Transparent Huge Pages
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References
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----------
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- `[1] directmap removal <https://lore.kernel.org/kvm/20260317141031.514-1-kalyazin@amazon.com/>`__
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- `[2] LUO <https://lore.kernel.org/kvm/20260728121138.1103610-1-tarunsahu@google.com/>`__
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