On Tue, 2021-01-26 at 03:18 -0500, Wei Huang wrote:
> Under the case of nested on nested (L0->L1->L2->L3), #GP triggered by
> SVM instructions can be hided from L1. Instead the hypervisor can
> inject the proper #VMEXIT to inform L1 of what is happening. Thus L1
> can avoid invoking the #GP workaround. For this reason we turns on
> guest VM's X86_FEATURE_SVME_ADDR_CHK bit for KVM running inside VM to
> receive the notification and change behavior.
>
> Similarly we check if vcpu is under guest mode before emulating the
> vmware-backdoor instructions. For the case of nested on nested, we
> let the guest handle it.
>
> Co-developed-by: Bandan Das <[email protected]>
> Signed-off-by: Bandan Das <[email protected]>
> Signed-off-by: Wei Huang <[email protected]>
> Tested-by: Maxim Levitsky <[email protected]>
> Reviewed-by: Maxim Levitsky <[email protected]>
> ---
> arch/x86/kvm/svm/svm.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f9233c79265b..83c401d2709f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -929,6 +929,9 @@ static __init void svm_set_cpu_caps(void)
>
> if (npt_enabled)
> kvm_cpu_cap_set(X86_FEATURE_NPT);
> +
> + /* Nested VM can receive #VMEXIT instead of triggering #GP */
> + kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK);
> }
>
> /* CPUID 0x80000008 */
> @@ -2198,6 +2201,11 @@ static int svm_instr_opcode(struct kvm_vcpu *vcpu)
>
> static int emulate_svm_instr(struct kvm_vcpu *vcpu, int opcode)
> {
> + const int guest_mode_exit_codes[] = {
> + [SVM_INSTR_VMRUN] = SVM_EXIT_VMRUN,
> + [SVM_INSTR_VMLOAD] = SVM_EXIT_VMLOAD,
> + [SVM_INSTR_VMSAVE] = SVM_EXIT_VMSAVE,
> + };
> int (*const svm_instr_handlers[])(struct vcpu_svm *svm) = {
> [SVM_INSTR_VMRUN] = vmrun_interception,
> [SVM_INSTR_VMLOAD] = vmload_interception,
> @@ -2205,7 +2213,14 @@ static int emulate_svm_instr(struct kvm_vcpu *vcpu,
> int opcode)
> };
> struct vcpu_svm *svm = to_svm(vcpu);
>
> - return svm_instr_handlers[opcode](svm);
> + if (is_guest_mode(vcpu)) {
> + svm->vmcb->control.exit_code = guest_mode_exit_codes[opcode];
> + svm->vmcb->control.exit_info_1 = 0;
> + svm->vmcb->control.exit_info_2 = 0;
> +
> + return nested_svm_vmexit(svm);
> + } else
> + return svm_instr_handlers[opcode](svm);
> }
>
> /*
> @@ -2239,7 +2254,8 @@ static int gp_interception(struct vcpu_svm *svm)
> * VMware backdoor emulation on #GP interception only handles
> * IN{S}, OUT{S}, and RDPMC.
> */
> - return kvm_emulate_instruction(vcpu,
> + if (!is_guest_mode(vcpu))
> + return kvm_emulate_instruction(vcpu,
> EMULTYPE_VMWARE_GP | EMULTYPE_NO_DECODE);
> } else
> return emulate_svm_instr(vcpu, opcode);
To be honest I expected the vmware backdoor fix to be in a separate patch,
but I see that Paulo already took these patches so I guess it is too late.
Anyway I am very happy to see this workaround merged, and see that bug
disappear forever.
Best regards,
Maxim Levitsky
