===Feature Introduction===
Linear-address masking (LAM) [1], modifies the checking that is applied to
*64-bit* linear addresses, allowing software to use of the untranslated address
bits for metadata and masks the metadata bits before using them as linear
addresses to access memory.
When the feature is virtualized and exposed to guest, it can be used for efficient
address sanitizers (ASAN) implementation and for optimizations in JITs and virtual
machines.
Regarding which pointer bits are masked and can be used for metadata, LAM has 2
modes:
- LAM_48: metadata bits 62:48, i.e. LAM width of 15.
- LAM_57: metadata bits 62:57, i.e. LAM width of 6.
* For user pointers:
CR3.LAM_U57 = CR3.LAM_U48 = 0, LAM is off;
CR3.LAM_U57 = 1, LAM57 is active;
CR3.LAM_U57 = 0 and CR3.LAM_U48 = 1, LAM48 is active.
* For supervisor pointers:
CR4.LAM_SUP =0, LAM is off;
CR4.LAM_SUP =1 with 5-level paging mode, LAM57 is active;
CR4.LAM_SUP =1 with 4-level paging mode, LAM48 is active.
The modified LAM canonicality check:
* LAM_S48 : [ 1 ][ metadata ][ 1 ]
63 47
* LAM_U48 : [ 0 ][ metadata ][ 0 ]
63 47
* LAM_S57 : [ 1 ][ metadata ][ 1 ]
63 56
* LAM_U57 + 5-lvl paging : [ 0 ][ metadata ][ 0 ]
63 56
* LAM_U57 + 4-lvl paging : [ 0 ][ metadata ][ 0...0 ]
63 56..47
Note:
1. LAM applies to only data address, not to instructions.
2. LAM identification of an address as user or supervisor is based solely on the
value of pointer bit 63 and does not depend on the CPL.
3. LAM doesn't apply to the writes to control registers or MSRs.
4. LAM masking applies before paging, so the faulting linear address in CR2
doesn't contain the metadata.
5 The guest linear address saved in VMCS doesn't contain metadata.
6. For user mode address, it is possible that 5-level paging and LAM_U48 are both
set, in this case, the effective usable linear address width is 48.
(Currently, only LAM_U57 is enabled in Linux kernel. [2])
===LAM KVM Design===
LAM KVM enabling includes the following parts:
- Feature Enumeration
LAM feature is enumerated by CPUID.7.1:EAX.LAM[bit 26].
If hardware supports LAM and host doesn't disable it explicitly (e.g. via
clearcpuid), LAM feature will be exposed to user VMM.
- CR4 Virtualization
LAM uses CR4.LAM_SUP (bit 28) to configure LAM on supervisor pointers.
Add support to allow guests to set the new CR4 control bit for guests to enable
LAM on supervisor pointers.
- CR3 Virtualization
LAM uses CR3.LAM_U48 (bit 62) and CR3.LAM_U57 (bit 61) to configure LAM on user
pointers.
Add support to allow guests to set two new CR3 non-address control bits for
guests to enable LAM on user pointers.
- Modified Canonicality Check and Metadata Mask
When LAM is enabled, 64-bit linear address may be tagged with metadata. Linear
address should be checked for modified canonicality and untagged in instruction
emulations and VMExit handlers when LAM is applicable.
LAM support in SGX enclave mode needs additional enabling and is not
included in this patch series.
This patch series depends on "governed" X86_FEATURE framework from Sean.
https://lore.kernel.org/kvm/20230217231022.816138-2-seanjc@xxxxxxxxxx/
This patch series depends on the patches of refactor of instruction emulation flags,
using flags to identify the access type of instructions, sent along with LASS patch series.
https://lore.kernel.org/kvm/20230719024558.8539-2-guang.zeng@xxxxxxxxx/
https://lore.kernel.org/kvm/20230719024558.8539-3-guang.zeng@xxxxxxxxx/
https://lore.kernel.org/kvm/20230719024558.8539-4-guang.zeng@xxxxxxxxx/
https://lore.kernel.org/kvm/20230719024558.8539-5-guang.zeng@xxxxxxxxx/
LAM QEMU patch:
https://lists.nongnu.org/archive/html/qemu-devel/2023-05/msg07843.html
LAM kvm-unit-tests patch:
https://lore.kernel.org/kvm/20230530024356.24870-1-binbin.wu@xxxxxxxxxxxxxxx/
===Test===
1. Add test cases in kvm-unit-test [3] for LAM, including LAM_SUP and LAM_{U57,U48}.
For supervisor pointers, the test covers CR4 LAM_SUP bits toggle, Memory/MMIO
access with tagged pointer, and some special instructions (INVLPG, INVPCID,
INVVPID), INVVPID cases also used to cover VMX instruction VMExit path.
For uer pointers, the test covers CR3 LAM bits toggle, Memory/MMIO access with
tagged pointer.
MMIO cases are used to trigger instruction emulation path.
Run the unit test with both LAM feature on/off (i.e. including negative cases).
Run the unit test in L1 guest with both LAM feature on/off.
2. Run Kernel LAM kselftests [2] in guest, with both EPT=Y/N.
3. Launch a nested guest.
All tests have passed in Simics environment.
[1] Intel ISE https://cdrdv2.intel.com/v1/dl/getContent/671368
Chapter Linear Address Masking (LAM)
[2] https://lore.kernel.org/all/20230312112612.31869-9-kirill.shutemov@xxxxxxxxxxxxxxx/
[3] https://lore.kernel.org/kvm/20230530024356.24870-1-binbin.wu@xxxxxxxxxxxxxxx/
---
Changelog
v10:
- Split out the patch "Use GENMASK_ULL() to define __PT_BASE_ADDR_MASK". [Sean]
- Split out the patch "Add & use kvm_vcpu_is_legal_cr3() to check CR3's legality". [Sean]
- Use "KVM-governed feature framework" to track if guest can use LAM. [Sean]
- Use emulation flags to describe the access instead of making the flag a command. [Sean]
- Split the implementation of vmx_get_untagged_addr() for LAM from emulator and kvm_x86_ops definition. [Per Sean's comment for LASS]
- Some improvement of implementation in vmx_get_untagged_addr(). [Sean]
v9:
https://lore.kernel.org/kvm/20230606091842.13123-1-binbin.wu@xxxxxxxxxxxxxxx/
Binbin Wu (7):
KVM: x86/mmu: Use GENMASK_ULL() to define __PT_BASE_ADDR_MASK
KVM: x86: Add & use kvm_vcpu_is_legal_cr3() to check CR3's legality
KVM: x86: Use KVM-governed feature framework to track "LAM enabled"
KVM: x86: Virtualize CR3.LAM_{U48,U57}
KVM: x86: Introduce get_untagged_addr() in kvm_x86_ops and call it in
emulator
KVM: VMX: Implement and wire get_untagged_addr() for LAM
KVM: x86: Untag address for vmexit handlers when LAM applicable
Robert Hoo (2):
KVM: x86: Virtualize CR4.LAM_SUP
KVM: x86: Expose LAM feature to userspace VMM
arch/x86/include/asm/kvm-x86-ops.h | 1 +
arch/x86/include/asm/kvm_host.h | 5 +++-
arch/x86/kvm/cpuid.c | 2 +-
arch/x86/kvm/cpuid.h | 8 ++++++
arch/x86/kvm/emulate.c | 2 +-
arch/x86/kvm/governed_features.h | 2 ++
arch/x86/kvm/kvm_emulate.h | 3 +++
arch/x86/kvm/mmu.h | 8 ++++++
arch/x86/kvm/mmu/mmu.c | 2 +-
arch/x86/kvm/mmu/mmu_internal.h | 1 +
arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
arch/x86/kvm/svm/nested.c | 4 +--
arch/x86/kvm/vmx/nested.c | 6 +++--
arch/x86/kvm/vmx/sgx.c | 1 +
arch/x86/kvm/vmx/vmx.c | 43 +++++++++++++++++++++++++++++-
arch/x86/kvm/vmx/vmx.h | 2 ++
arch/x86/kvm/x86.c | 15 +++++++++--
arch/x86/kvm/x86.h | 2 ++
18 files changed, 97 insertions(+), 12 deletions(-)
base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
prerequisite-patch-id: 3467bc611ce3774ba481ab72e187eba47000c01b
prerequisite-patch-id: 1bf4c9da384b39c92c21c467a5c6ed0d306ec266
prerequisite-patch-id: 226fd3d9a09ef80a5b8001a3bdc6fbf2c23d2a88
prerequisite-patch-id: 0c31cc0dec011d7e22efde1f7dde9847c86024d8
prerequisite-patch-id: f487db8bc77007679f4b0e670a9e487c1f63fcfe
--
2.25.1
