On Sat, 11 Apr 2020 at 01:47, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > On Fri, Apr 10, 2020 at 05:50:35PM +0200, Paolo Bonzini wrote: > > On 10/04/20 17:35, Sean Christopherson wrote: > > > IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it > > > needs to be moved below the #MC handling and below > > > > > > if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY)) > > > return; > > > > Why? It cannot run in any of those cases, since the vmx->exit_reason > > won't match. > > #MC and consistency checks should have "priority" over everything else. > That there isn't actually a conflict is irrelevant IMO. And it's something > that will likely confuse newbies (to VMX and/or KVM) as it won't be obvious > that the motivation was to shave a few cycles, e.g. versus some corner case > where the fastpath handling does something meaningful even on failure. > > > > KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's > > > not obvious that a generic fastpath call can safely run before > > > vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call. > > > > Not KVM, rather vmx.c. You're right about a generic fastpath, but in > > this case kvm_irq_delivery_to_apic_fast is not touching VMX state; even > > if you have a self-IPI, the modification of vCPU state is only scheduled > > here and will happen later via either kvm_x86_ops.sync_pir_to_irr or > > KVM_REQ_EVENT. > > I think what I don't like is that the fast-IPI code is buried in a helper > that masquerades as a generic fastpath handler. If that's open-coded in > vmx_vcpu_run(), I'm ok with doing the fast-IPI handler immediately after > the failure checks. > > And fast-IPI aside, the code could use a bit of optimization to prioritize > successful VM-Enter, which would slot in nicely as a prep patch. Patches > (should be) following. Thanks for v3. :) > > IMO, this is more logically correct: > > vmx->exit_reason = vmcs_read32(VM_EXIT_REASON); > if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)) > kvm_machine_check(); > > if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY)) > return EXIT_FASTPATH_NONE; > > if (!is_guest_mode(vcpu) && vmx->exit_reason == EXIT_REASON_MSR_WRITE) > exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu); > else > exit_fastpath = EXIT_FASTPATH_NONE; > > And on my system, the compiler hoists fast-IPI above the #MC, e.g. moving > the fast-IPI down only adds a single macrofused uop, testb+jne for > FAILED_VMENTERY, to the code path. > > 0xffffffff81067d1d <+701>: vmread %rax,%rax > 0xffffffff81067d20 <+704>: ja,pt 0xffffffff81067d2d <vmx_vcpu_run+717> > 0xffffffff81067d23 <+707>: pushq $0x0 > 0xffffffff81067d25 <+709>: push %rax > 0xffffffff81067d26 <+710>: callq 0xffffffff81071790 <vmread_error_trampoline> > 0xffffffff81067d2b <+715>: pop %rax > 0xffffffff81067d2c <+716>: pop %rax > 0xffffffff81067d2d <+717>: test %eax,%eax > 0xffffffff81067d2f <+719>: mov %eax,0x32b0(%rbp) > 0xffffffff81067d35 <+725>: js 0xffffffff81067d5a <vmx_vcpu_run+762> > 0xffffffff81067d37 <+727>: testb $0x20,0x2dc(%rbp) > 0xffffffff81067d3e <+734>: jne 0xffffffff81067d49 <vmx_vcpu_run+745> > 0xffffffff81067d40 <+736>: cmp $0x20,%eax > 0xffffffff81067d43 <+739>: je 0xffffffff810686d4 <vmx_vcpu_run+3188> <-- fastpath handler > 0xffffffff81067d49 <+745>: xor %ebx,%ebx > 0xffffffff81067d4b <+747>: jmpq 0xffffffff81067e65 <vmx_vcpu_run+1029>
