On Wed, May 27, 2026 at 07:03:25PM -0500, Michael Roth wrote: > This patchset is also available at: > > https://github.com/amdese/qemu/commits/snp-inplace-rfc1 > > which is in turn based on the following series: > > [PATCH 0/4] "guest_memfd: Fix handling for conversions of MMIO ranges" > https://lists.gnu.org/archive/html/qemu-devel/2026-05/msg07547.html > > > OVERVIEW > -------- > > This series adds guest_memfd support for in-place conversion of memory > between private/shared, and enables it for SEV-SNP guests. It is based > on recently-added kernel support for mmap()-able guest_memfd > instances[1], which allow it to be used for shared memory, and the > following patchset[2], which adds additional guest_memfd interfaces to > allow it to be used to perform in-place conversion: > > "[PATCH v7 00/42] guest_memfd: In-place conversion support" > > https://lore.kernel.org/kvm/[email protected]/ > > That series also introduces a new 'vm_memory_attributes' KVM > module option, which sets whether memory attributes are tracked > VM-wide by KVM (vm_memory_attributes=1: the existing 'legacy' mode), > or per-guest_memfd instance (vm_memory_attributes=0: the new mode > which allows for in-place conversion). The latter is intended to > eventually deprecate the legacy mode, at which point in-place > conversion would become the primarily-supported mode. > > > MOTIVATION > ---------- > > Today, SEV-SNP guests (and other CoCo VM types using guest_memfd) keep > shared and private memory on separate physical backings: a userspace > memory-backend object for shared pages, and a kernel-allocated > guest_memfd file descriptor for private pages. KVM_SET_MEMORY_ATTRIBUTES > flips which backing the guest sees for a given GPA range, and the old > backing is typically discarded / hole-punched on conversion to avoid > doubled memory usage.
Hi Michael, I am giving this a go on Arm CCA on top of Ackerley's KVM patches. When convert-in-place is switched on I think that the post conversion hook should not trigger discard+hole-punch since now guest-memfd _is_ the memory back-end but it looks like there is no guard in place against that (I noticed that ram_block_discard_range() triggers a hole-punch in kvm_post_convert_section() - when the CCA guest first requests a KVM_EXIT_MEMORY_FAULT to convert to private). It is a question really. Thanks, Lorenzo > That model works, but has a number of downsides that impact certain > use-cases: > > - Each conversion involves discarding pages on one side and faulting > them in on the other, which incurs allocation overheads in the > host kernel for every conversion. > > - Some use-cases, like pKVM[3], rely on memory isolation rather than > encryption and rely on in-place conversion to pass through things > like secured framebuffer memory without needing to bounce data > through separate shared/private HPAs, which would introduce > unacceptable latency for that sort of workload. > > - Hugetlb support[4] for guest_memfd will rely on it, since things like > 1GB hugepages with a mix of shared/private sub-ranges would generally > require 2 1GB hugetlb pages to remain available to handle shared vs. > private accesses, which quickly causes doubling of guest memory usage. > > Recent kernel work[2] makes guest_memfd mmap()-able and lets the *same* > physical pages be used for both shared and private states for a given > GPA range, allowing the above pitfalls to be naturally avoided. > > This series wires that support up in QEMU. > > > DESIGN > ------ > > A new dedicated memory backend, memory-backend-guest-memfd, allocates > its memory via a guest_memfd file descriptor obtained from KVM with > the GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_INIT_SHARED flags. The fd > is mmap()ed so userspace can access pages directly while they are in > the shared state. For a normal/non-confidential VM, this backend can > be used in a similar fashion as the existing memory-backend-memfd. > > For confidential VMs, a new 'convert-in-place' flag is added to switch > on in-place conversion support. When running in this mode, the user > *MUST* use memory-backend-guest-memfd for backing guest RAM. A new > RAM_GUEST_MEMFD_SHARED RAMBlock flag is added to track/enforce the > dependency. Additionally, QEMU is modified to use mmap()-able > guest_memfd and set this flag for other cases where it allocates RAM > internally. As a result, block->fd will generally always a guest_memfd, > and when RAM_GUEST_MEMFD_SHARED is set then that block->fd will be > qemu_dup()'d as the FD handle for private memory is well (which is > currently what block->guest_memfd point to). This allows the prior > non-in-place handling around block->guest_memfd to be kept mostly > unchanged. > > When running with convert-in-place=true, shared/private conversions > are no longer handled directly by KVM, but instead by a new guest_memfd > ioctl, KVM_SET_MEMORY_ATTRIBUTES2, which purposely provides similar > naming/implementation to the KVM_SET_MEMORY_ATTRIBUTES KVM ioctl that > it replaces. This series adds handling to route conversion requests to > the appropriate ioctls based on whether or not in-place conversion is > enabled. > > Since guest_memfd ioctls need to be called against the specific > guest_memfd inode associated with each memory slot/region, some > refactoring is needed to handle conversions on a per-section. Much of > that is inherited from the bugfix series this patchset is based on top > of, which adds the initial logic for handling multiple sections within > a range that gets heavily re-used here. > > > USAGE > ----- > > After applying this series against a kernel with the RFC patches above > present, an SEV-SNP guest can be started with in-place conversion via: > > qemu-system-x86_64 \ > -machine q35,confidential-guest-support=sev0,memory-backend=ram0 \ > -object memory-backend-guest-memfd,id=ram0,size=8G,share=on \ > -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,\ > convert-in-place=on \ > ... > > The new memory-backend-guest-memfd can also be used by normal VMs: > > qemu-system-x86_64 \ > -machine q35,memory-backend=ram0 \ > -object memory-backend-guest-memfd,id=ram0,size=8G,share=on \ > ... > > This is mainly only useful atm for testing, but in the future there may > be more use-cases around using guest_memfd as a general-purpose backend > for non-confidential VMs, so it is intended to work in this manner as > well. > > > NOTES/TODO > ---------- > > - the CPR handling to support resetting of confidential VMs is > currently disabled when in-place conversion is enabled. > - TDX testing would be great, in theory it can be enabled with this > series (similarly to the top patch) but I'm not sure if there are > other special requirements before we can switch it on. > - kernel patches are still in-flight, but fairly mature at this point > and nearing upstream > > > REFERENCES > ---------- > > [1] https://lore.kernel.org/kvm/[email protected]/ > [2] > https://lore.kernel.org/kvm/[email protected]/ > [3] https://www.youtube.com/watch?v=MMfAGNW9RVg > [4] 1GB hugetlb v2 > > > Thoughts, feedback, and testing are very much appreciated. > > Thanks, > > Mike > > > ---------------------------------------------------------------- > Michael Roth (12): > accel/kvm: Decouple guest_memfd checks from memory attribute checks > hostmem: Introduce dedicated memory backend for guest_memfd > linux-headers: Update headers for v7 of in-place conversion kernel > support > accel/kvm: Add CGS option to control in-place conversion support > system/memory: Re-use memory-backend-guest-memfd inode for private > memory > system/memory: Default to guest_memfd for RAM for in-place conversion > accel/kvm: Move post-conversion updates to a separate helper > accel/kvm: Re-order attribute notifications for in-place conversion > accel/kvm: Support shared/private conversions via guest_memfd ioctls > accel/kvm: Don't default to private attributes for in-place conversion > i386/sev: Update SNP_LAUNCH_UPDATE for in-place conversion > i386/sev: Allow in-place conversion for SEV-SNP guests > > accel/kvm/kvm-all.c | 286 +++++++++++-- > accel/stubs/kvm-stub.c | 9 +- > backends/confidential-guest-support.c | 25 ++ > backends/hostmem-guest-memfd.c | 93 +++++ > backends/meson.build | 1 + > include/standard-headers/drm/drm_fourcc.h | 28 +- > include/standard-headers/linux/const.h | 18 + > include/standard-headers/linux/ethtool.h | 28 +- > include/standard-headers/linux/input-event-codes.h | 13 + > include/standard-headers/linux/pci_regs.h | 71 +++- > include/standard-headers/linux/typelimits.h | 8 + > include/standard-headers/linux/virtio_ring.h | 5 +- > include/standard-headers/linux/virtio_rtc.h | 237 +++++++++++ > include/standard-headers/linux/vmclock-abi.h | 20 + > include/system/confidential-guest-support.h | 14 + > include/system/hostmem.h | 1 + > include/system/kvm.h | 3 +- > include/system/memory.h | 8 +- > linux-headers/asm-arm64/kvm.h | 1 + > linux-headers/asm-arm64/unistd_64.h | 1 + > linux-headers/asm-generic/unistd.h | 5 +- > linux-headers/asm-loongarch/kvm.h | 5 + > linux-headers/asm-loongarch/kvm_para.h | 1 + > linux-headers/asm-loongarch/unistd_64.h | 2 + > linux-headers/asm-mips/unistd_n32.h | 1 + > linux-headers/asm-mips/unistd_n64.h | 1 + > linux-headers/asm-mips/unistd_o32.h | 1 + > linux-headers/asm-powerpc/unistd_32.h | 1 + > linux-headers/asm-powerpc/unistd_64.h | 1 + > linux-headers/asm-riscv/kvm.h | 11 +- > linux-headers/asm-riscv/ptrace.h | 37 ++ > linux-headers/asm-riscv/unistd_32.h | 1 + > linux-headers/asm-riscv/unistd_64.h | 1 + > linux-headers/asm-s390/unistd_32.h | 446 > --------------------- > linux-headers/asm-s390/unistd_64.h | 1 + > linux-headers/asm-x86/kvm.h | 21 +- > linux-headers/asm-x86/unistd_32.h | 1 + > linux-headers/asm-x86/unistd_64.h | 1 + > linux-headers/asm-x86/unistd_x32.h | 1 + > linux-headers/linux/const.h | 18 + > linux-headers/linux/iommufd.h | 48 +++ > linux-headers/linux/kvm.h | 62 ++- > linux-headers/linux/mshv.h | 4 +- > linux-headers/linux/psp-sev.h | 2 +- > linux-headers/linux/stddef.h | 4 + > linux-headers/linux/vduse.h | 85 +++- > linux-headers/linux/vfio.h | 30 +- > qapi/qom.json | 35 +- > qemu-options.hx | 5 + > system/memory.c | 22 +- > system/physmem.c | 50 ++- > target/i386/sev.c | 12 +- > 52 files changed, 1253 insertions(+), 533 deletions(-) > create mode 100644 backends/hostmem-guest-memfd.c > create mode 100644 include/standard-headers/linux/typelimits.h > create mode 100644 include/standard-headers/linux/virtio_rtc.h > delete mode 100644 linux-headers/asm-s390/unistd_32.h >
