[PATCH v2 0/2] Add support for cmpxchg16b emulation

[Sairaj Kodilkar <sarunkod@amd.com>]

This series adds 128-bit compare-exchange support needed for KVM to
emulate guest cmpxchg16b instructions.

The AMD IOMMU driver updates 256-bit device table entries with two
128-bit cmpxchg operations.  For hardware-accelerated vIOMMU, QEMU traps
those DTE accesses to install nested page tables [1].  KVM must emulate
guest cmpxchg16b on the trapped MMIO regions; without that, DTE access
emulation fails.

Patch 1: extends the x86 user CMPXCHG helpers with
__try_cmpxchg128_user_asm() (cmpxchg16b on x86-64), wired into
unsafe_try_cmpxchg_user().

Patch 2: extends KVM CMPXCHG8B emulation to 16-byte operands (REX.W=1) and
uses the new helper for atomic 16-byte compare-exchange on guest RAM via
emulator_cmpxchg_emulated().

You can find the KUT to test this series at [2].

[1] https://github.com/AMDESE/qemu-iommu/blob/wip/for_iommufd_hw_queue-v8_amd_viommu_20260106/hw/i386/amd_viommu.c#L517
[2] https://lore.kernel.org/kvm/20260706062153.346-1-sarunkod@amd.com/T/#u

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Base: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6

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Sairaj Kodilkar (2):
  x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands
  KVM: x86: Add support for cmpxchg16b emulation

 arch/x86/include/asm/uaccess.h | 56 +++++++++++++++++++++++++++++++++-
 arch/x86/kvm/emulate.c         | 50 ++++++++++++++++++++----------
 arch/x86/kvm/kvm_emulate.h     |  6 ++++
 arch/x86/kvm/x86.c             |  7 ++++-
 4 files changed, 101 insertions(+), 18 deletions(-)

-- 
2.34.1