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Subject[RFC PATCH 0/7] KVM: SVM: move MSR_IA32_SPEC_CTRL save/restore to assembly
Date
On the Intel side, restoration of the guest's IA32_SPEC_CTRL is done
as late as possible before vmentry, with the comment:

* IMPORTANT: To avoid RSB underflow attacks and any other nastiness,
* there must not be any returns or indirect branches between this code
* and vmentry.

On AMD, there is no need to avoid returns or indirect branches between
wrmsr and vmrun because Linux doesn't use IBRS; however, restoration
of the host IA32_SPEC_CTRL value is definitely way too late. With
respect to the user/kernel boundary, AMD says, "If software chooses to
toggle STIBP (e.g., set STIBP on kernel entry, and clear it on kernel
exit), software should set STIBP to 1 before executing the return thunk
training sequence." Assuming the same requirements apply to the guest/host
boundary, KVM does not respect this recommendation: the return thunk
training sequence is in vmenter.S, quite close to the VM-exit, while
the host's IA32_SPEC_CTRL value is only restored much later for hosts
without V_SPEC_CTRL.

In the absence of clarifications for AMD, move all the SPEC_CTRL
handling to assembly code and, in passing, also make the Intel and AMD
code a bit more similar to each other.

Patches 1-2 are the Intel side, which is just a cleanup.

Patch 3 prepares for adding asm-offsets.c entries in arch/x86/kvm/svm/svm.h,
and patches 4-5 are a similar cleanup to the earlier VMX ones.

Patch 6 is the bulk of the change, and finally patch 7 removes now
dead code in asm/spec-ctrl.h and arch/x86/kernel/.

This is RFC because I haven't tested SEV-ES or 32-bit yet.

Paolo

Paolo Bonzini (7):
KVM: VMX: remove regs argument of __vmx_vcpu_run
KVM: VMX: more cleanups to __vmx_vcpu_run
KVM: SVM: extract VMCB accessors to a new file
KVM: SVM: replace argument of __svm_vcpu_run with vcpu_svm
KVM: SVM: adjust register allocation for __svm_vcpu_run
KVM: SVM: move MSR_IA32_SPEC_CTRL save/restore to assembly
x86, KVM: remove unnecessary argument to x86_virt_spec_ctrl and
callers

arch/x86/include/asm/spec-ctrl.h | 10 +-
arch/x86/kernel/asm-offsets.c | 8 ++
arch/x86/kernel/cpu/bugs.c | 15 +--
arch/x86/kvm/svm/avic.c | 1 +
arch/x86/kvm/svm/nested.c | 1 +
arch/x86/kvm/svm/sev.c | 1 +
arch/x86/kvm/svm/svm.c | 39 +++---
arch/x86/kvm/svm/svm.h | 204 +-----------------------------
arch/x86/kvm/svm/svm_onhyperv.c | 1 +
arch/x86/kvm/svm/vmcb.h | 211 +++++++++++++++++++++++++++++++
arch/x86/kvm/svm/vmenter.S | 164 ++++++++++++++++++------
arch/x86/kvm/vmx/nested.c | 3 +-
arch/x86/kvm/vmx/vmenter.S | 92 ++++++--------
arch/x86/kvm/vmx/vmx.c | 3 +-
arch/x86/kvm/vmx/vmx.h | 3 +-
15 files changed, 419 insertions(+), 337 deletions(-)
create mode 100644 arch/x86/kvm/svm/vmcb.h

--
2.31.1

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