===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.
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, for the purposes of LAM, depend on the CPL.
3. 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. [2]
4. When VM exit, the page faulting linear address saved in VMCS field is clean,
i.e. metadata cleared with canonical form.
===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 masking on supervisor pointers.
CR4.LAM_SUP is allowed to be set if vCPU supports LAM, including in nested guest.
CR4.LAM_SUP is allowed to be set even not in 64-bit mode, but it will not take
effect since LAM only applies to 64-bit linear address.
Change of CR4.LAM_SUP bit is intercepted to avoid vmread every time when KVM
fetches its value, with the expectation that guest won't toggle the bit frequently.
Hardware is not required to do TLB flush when CR4.LAM_SUP toggled, so KVM doesn't
need to emulate TLB flush based on it.
- CR3 Virtualization
LAM uses CR3.LAM_U48 (bit 62) and CR3.LAM_U57 (bit 61) to configure LAM masking
for user mode pointers.
When EPT is on:
CR3 is fully under control of guest, guest LAM is thus transparent to KVM.
When EPT is off (shadow paging):
* KVM needs to handle guest CR3.LAM_U48 and CR3.LAM_U57 toggles.
The two bits are allowed to be set in CR3 if vCPU supports LAM.
The two bits should be kept as they are in the shadow CR3.
* Perform GFN calculation from guest CR3/PGD generically by extracting the
maximal base address mask.
* Leave LAM bits in root.pgd to force a new root for a CR3+LAM combination.
To be generic, introduce a field 'cr3_ctrl_bits' in kvm_vcpu_arch to record
the bits used to control supported features related to CR3 (e.g. LAM).
- 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 (i.e. metadata
bits should be masked by sign-extending the bit 47 or bit 56) in instruction
emulations and VMExit handlings when LAM is applicable.
LAM inside nested guest is supported by this patch series.
LAM inside SGX enclave mode is NOT supported by this patch series.
The patch series is based on linux kernel v6.3-rc4, depends on two patches:
- One from Kiril for LAM feature and flag definitions[3].
- The other is a bug fix sent out speperatly[4].
The patch series organized as following:
Patch 1/2: CR4/CR3 virtualization
Patch 3: Implementation of untag_addr
Patch 4: Untag address when LAM applicable
Patch 5: Expose LAM feature to userspace VMM
The corresponding QEMU patch:
https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg08036.html
===Unit Test===
1. Add a kvm-unit-test [5] for LAM, including LAM_SUP and LAM_{U57,U48}.
For supervisor mode, this test covers CR4 LAM_SUP bits toggle, Memory/MMIO
access with tagged pointer, and some special instructions (INVLPG, INVPCID,
INVVPID), INVVIID cases also used to cover VMX instruction VMExit path.
For uer mode, this 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).
2. Run Kernel LAM kselftests 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] Thus currently, LAM kernel enabling patch only enables LAM_U57.
https://lore.kernel.org/lkml/20230123220500.21077-1-kirill.shutemov@xxxxxxxxxxxxxxx/
[3] https://lore.kernel.org/lkml/20230123220500.21077-4-kirill.shutemov@xxxxxxxxxxxxxxx/
[4] https://lore.kernel.org/kvm/20230404032502.27798-1-binbin.wu@xxxxxxxxxxxxxxx/
[5] https://lore.kernel.org/kvm/20230319083732.29458-1-binbin.wu@xxxxxxxxxxxxxxx/
---
Changelog
v6 --> v7:
- Changes to CR3 virtualization when EPT off
* Leave LAM bits in root.pgd to force a new root for a CR3+LAM combination. (Sean)
* Perform GFN calculation from guest CR3/PGD generically by extracting the maximal
base address mask. (Sean)
- Remove derefence of ctxt->vcpu in the emulator. (Sean)
- Fix a bug in v6, which hardcoded "write" to "false" by mistake in linearize(). (Chao Gao)
- Add Chao Gao's reviwed-by in Patch 5.
- Add Xuelian Guo's tested-by in the patch set.
- Seperate cleanup patches from the patch set.
v5 --> v6:
Add Patch 2 to fix the check of 64-bit mode.
Add untag_addr() to kvm_x86_ops to hide vendor specific code.
Simplify the LAM canonicality check per Chao's suggestion.
Add cr3_ctrl_bits to kvm_vcpu_arch to simplify cr3 invalidation/extract/mask (Chao Gao)
Extend the patchset scope to include nested virtualization and SGX ENCLS handling.
- Add X86_CR4_LAM_SUP in cr4_fixed1_update for nested vmx. (Chao Gao)
- Add SGX ENCLS VMExit handling
- Add VMX insturction VMExit handling
More descriptions in cover letter.
Add Chao's reviwed-by on Patch 4.
Add more test cases in kvm-unit-test.
v4 --> v5:
Reorder and melt patches surround CR3.LAM bits into Patch 3 of this
version.
Revise Patch 1's subject and description
Drop Patch 3
Use kvm_read_cr4_bits() instead of kvm_read_cr4()
Fix: No need to untag addr when write to msr, it should be legacy canonical check
Rename kvm_is_valid_cr3() --> kvm_vcpu_is_valid_cr3(), and update some call
sites of kvm_vcpu_is_valid_cr3() to use kvm_is_valid_cr3().
Other refactors and Miscs.
v3 --> v4:
Drop unrelated Patch 1 in v3 (Binbin, Sean, Xiaoyao)
Intercept CR4.LAM_SUP instead of pass through to guest (Sean)
Just filter out CR3.LAM_{U48, U57}, instead of all reserved high bits
(Sean, Yuan)
Use existing __canonical_address() helper instead write a new one (Weijiang)
Add LAM handling in KVM emulation (Yu, Yuan)
Add Jingqi's reviwed-by on Patch 7
Rebased to Kirill's latest code, which is 6.2-rc1 base.
v2 --> v3:
As LAM Kernel patches are in tip tree now, rebase to it.
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/
v1 --> v2:
1. Fixes i386-allyesconfig build error on get_pgd(), where
CR3_HIGH_RSVD_MASK isn't applicable.
(Reported-by: kernel test robot <lkp@xxxxxxxxx>)
2. In kvm_set_cr3(), be conservative on skip tlb flush when only LAM bits
toggles. (Kirill)
Binbin Wu (2):
KVM: x86: Introduce untag_addr() in kvm_x86_ops
KVM: x86: Untag address when LAM applicable
Robert Hoo (3):
KVM: x86: Virtualize CR4.LAM_SUP
KVM: x86: Virtualize CR3.LAM_{U48,U57}
KVM: x86: Expose LAM feature to userspace VMM
arch/x86/include/asm/kvm-x86-ops.h | 1 +
arch/x86/include/asm/kvm_host.h | 14 +++++-
arch/x86/kvm/cpuid.c | 2 +-
arch/x86/kvm/cpuid.h | 5 +++
arch/x86/kvm/emulate.c | 23 +++++++---
arch/x86/kvm/kvm_emulate.h | 2 +
arch/x86/kvm/mmu.h | 5 +++
arch/x86/kvm/mmu/mmu.c | 6 ++-
arch/x86/kvm/mmu/mmu_internal.h | 1 +
arch/x86/kvm/mmu/paging_tmpl.h | 6 ++-
arch/x86/kvm/mmu/spte.h | 2 +-
arch/x86/kvm/svm/svm.c | 7 +++
arch/x86/kvm/vmx/nested.c | 8 +++-
arch/x86/kvm/vmx/sgx.c | 1 +
arch/x86/kvm/vmx/vmx.c | 69 +++++++++++++++++++++++++++++-
arch/x86/kvm/vmx/vmx.h | 2 +
arch/x86/kvm/x86.c | 14 +++++-
arch/x86/kvm/x86.h | 2 +
18 files changed, 155 insertions(+), 15 deletions(-)
base-commit: 197b6b60ae7bc51dd0814953c562833143b292aa
prerequisite-patch-id: 883dc8f73520b47a6c3690c1704f2e85a2713e4f
prerequisite-patch-id: cf5655ce89a2390cd29f33c57a4fc307a6045f62
--
2.25.1
