[PATCH v4 0/9] Linear Address Masking (LAM) KVM Enabling

Robert Hoo
2023-02-09T02:41:12Z

===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 (upper) bits for metadata.
As for which upper bits of linear address can be borrowed, LAM has 2 modes:
LAM_48 (bits 62:48, i.e. LAM width of 15) and LAM_57 (bits 62:57, i.e. LAM
width of 6), controlled by these new bits: CR3[62] (LAM_U48), CR3[61]
(LAM_U57), and CR4[28] (LAM_SUP).

* LAM_U48 and LAM_U57 bits controls LAM for user mode address. I.e. if
CR3.LAM_U57 = 1, LAM57 is applied; if CR3.LAM_U48 = 1 and CR3.LAM_U57 = 0,
LAM48 is applied.
* LAM_SUP bit, combined with paging mode (4-level or 5-level), determines
LAM status for supervisor mode address. I.e. when CR4.LAM_SUP =1, 4-level
paging mode will have LAM48 for supervisor mode address while 5-level paging
will have LAM57.

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, i.e. bit
56:47 is reserved by LAM. [2]
4. When VM exit, the problematic address saved in VMCS field is clean, i.e.
metadata cleared with canonical form.

===LAM KVM Design===

Intercept CR4.LAM_SUP by KVM, to avoid read VMCS field every time, with
expectation that guest won't toggle this bit frequently.

Under EPT mode, CR3 is fully under guest control, guest LAM is thus transparent to
KVM. Nothing more need to do.

For Shadow paging (EPT = off), KVM need to handle guest CR3.LAM_U48 and CR3.LAM_U57
toggles.

[1] ISE Chap10 https://cdrdv2.intel.com/v1/dl/getContent/671368 (Section 10.6 VMX interaction)
[2] Thus currently, Kernel enabling patch only enables LAM_U57. https://lore.kernel.org/lkml/20230123220500.21077-1-kirill.shutemov@linux.intel.com/

---
Changelog
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@intel.com>)
2. In kvm_set_cr3(), be conservative on skip tlb flush when only LAM bits
toggles. (Kirill)

Robert Hoo (9):
KVM: x86: Intercept CR4.LAM_SUP when LAM feature is enabled in guest
KVM: x86: MMU: Clear CR3 LAM bits when allocate shadow root
[Trivial] KVM: x86: MMU: Commets update
KVM: x86: MMU: Integrate LAM bits when build guest CR3
KVM: x86: Untag LAM bits when applicable
KVM: x86: When judging setting CR3 valid or not, consider LAM bits
KVM: x86: When guest set CR3, handle LAM bits semantics
KVM: x86: LAM: Expose LAM CPUID to user space VMM
KVM: x86: emulation: Apply LAM when emulating data access

arch/x86/include/asm/kvm_host.h | 3 ++-
arch/x86/kvm/cpuid.c | 2 +-
arch/x86/kvm/emulate.c | 6 +++++
arch/x86/kvm/mmu.h | 5 ++++
arch/x86/kvm/mmu/mmu.c | 11 ++++++--
arch/x86/kvm/vmx/vmx.c | 6 ++++-
arch/x86/kvm/x86.c | 38 ++++++++++++++++++++++----
arch/x86/kvm/x86.h | 47 +++++++++++++++++++++++++++++++++
8 files changed, 108 insertions(+), 10 deletions(-)

base-commit: 03334443640f226f56f71b5dfa3b1be6d4a1a1bc
(https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git lam)
--
2.31.1