From: Tian, Kevin Sent: Thursday, December 30, 2021 3:05 PM
the new change is like below.
static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu) { /* * Save xfd_err to guest_fpu before interrupt is enabled, so the * guest value is not clobbered by the host activity before the guest * has chance to consume it. * * Since trapping #NM is started when xfd write interception is * disabled, using this flag to guard the saving operation. This * implies no-op for a non-xfd #NM due to L1 interception. * * Queuing exception is done in vmx_handle_exit. */ if (vcpu->arch.xfd_no_write_intercept) rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err); }
in the final series it will first check vcpu->arch.guest_fpu.fpstate->xfd before the disable interception patch is applied and then becomes the above form, similar to your suggestion on vmx_update_exception_bitmap().
whether to check msr_bitmap vs. an extra flag is an orthogonal open.
Then:
handle_exception_nmi(struct kvm_vcpu *vcpu) { ... if (is_machine_check(intr_info) || is_nmi(intr_info)) return 1; /* handled by handle_exception_nmi_irqoff() */
/* * Queue the exception here instead of in handle_nm_fault_irqoff(). * This ensures the nested_vmx check is not skipped so vmexit can * be reflected to L1 (when it intercepts #NM) before reaching this * point. */ if (is_nm_fault(intr_info)) { kvm_queue_exception(vcpu, NM_VECTOR); return 1; }
... }
Then regarding to test non-AMX nested #NM usage, it might be difficult to trigger it from modern OS. As commented by Linux #NM handler, it's expected only for XFD or math emulation when fpu is missing. So we plan to run a selftest in L1 which sets CR0.TS and then touch fpu registers. and for L1 kernel we will run two binaries with one trapping #NM and the other not.
We have verified this scenario and didn't find problem.
Basically the selftest is like below:
guest_code() { cr0 = read_cr0(); cr0 |= X86_CR0_TS; write_cr0(cr0);
asm volatile("fnop"); }
guest_nm_handler() { cr0 = read_cr0(); cr0 &= ~X86_CR0_TS; write_cr0(cr0); }
We run the selftest in L1 to create a nested scenario.
When L1 intercepts #NM:
(L2) fnop (L0) #NM vmexit (L0) reflect a virtual vmexit (reason #NM) to L1 (L1) #NM vmexit (L1) queue #NM exception to L2 (L2) guest_nm_handler() (L2) fnop (succeed)
When L1 doesn't intercept #NM: (L2) fnop (L0) #NM vmexit (L0) queue #NM exception to L2 (L2) guest_nm_handler() (L2) fnop (succeed)
Please suggest if any more test is necessary here.
Thanks Kevin