[PATCH v2 0/7] KVM: VMX/SVM: use the same SPEC_CTRL assembly code

Paolo Bonzini
2026-04-27T10:58:54Z

As discussed in the APX thread, this series moves the SPEC_CTRL
handling for VMX entirely to vmenter.S, and while at it improves
the code reuse between SVM and VMX.

The prototype of __vmx_vcpu_run() and __svm_vcpu_run() becomes
the same, with a set of bit flags for the second argument (which
can be augmented with an extra bit for whether to save/restore APX).
The register allocation also becomes very similar, with %edi/%rdi
pointing to the vmx (resp. svm) argument (patches 1-4).

(In fact, most of this was already done at the time of commit 9f2febf3f04d
and predecessors, but I had never tested it and submitted it... because
well, assembly is scary).

Thanks to this, the code to restore the guest SPEC_CTRL value is the
same up to the register names and can be dropped into vmx/vmenter.S.
On the vmexit side the only change required is support for
X86_FEATURE_KERNEL_IBRS, which is a single ALTERNATIVE macro.

Paolo

patch 2:
- fix "Reload @vmx to RAX" comment [Binbin]

patch 4:
- fix polarity of KVM_ENTER_SAVE_SPEC_CTRL in SVM, which would
have caused problems when the macros are reused for VMX
- adjust prototypes of __svm_vcpu_run and __svm_sev_es_vcpu_run

patch 7:
- fix order of ALTERNATIVE arguments for X86_FEATURE_KERNEL_IBRS.
Add comment
- Leave @flags and @vmx on the stack during RESTORE_HOST_SPEC_CTRL_BODY.
[Uros, Binbin]

Paolo Bonzini (7):
KVM: VMX: remove regs argument of __vmx_vcpu_run
KVM: VMX: more cleanups to __vmx_vcpu_run
KVM: SVM: prepare for making SPEC_CTRL switch common with VMX
KVM: SVM: adopt the same VMX_RUN_* flags as VMX
KVM: SVM: extract RESTORE_*_SPEC_CTRL_BODY out of svm/vmenter.S
KVM: VMX: switch to RESTORE_GUEST_SPEC_CTRL_BODY
KVM: VMX: replace vmx_spec_ctrl_restore_host with
RESTORE_HOST_SPEC_CTRL_BODY

arch/x86/kvm/kvm-asm-offsets.c | 1 +
arch/x86/kvm/svm/svm.c | 14 ++-
arch/x86/kvm/svm/svm.h | 4 +-
arch/x86/kvm/svm/vmenter.S | 103 +++++--------------
arch/x86/kvm/vmenter.h | 80 +++++++++++++++
arch/x86/kvm/vmx/run_flags.h | 9 --
arch/x86/kvm/vmx/vmenter.S | 176 ++++++++++++++-------------------
arch/x86/kvm/vmx/vmx.c | 39 ++------
arch/x86/kvm/vmx/vmx.h | 7 +-
9 files changed, 199 insertions(+), 234 deletions(-)
create mode 100644 arch/x86/kvm/vmenter.h
delete mode 100644 arch/x86/kvm/vmx/run_flags.h

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2.52.0