On Mon, Sep 23, 2019 at 11:57:57AM +0200, Paolo Bonzini wrote: > On 23/09/19 11:31, Vitaly Kuznetsov wrote: > > +#ifdef CONFIG_RETPOLINE > > + if (exit_reason == EXIT_REASON_MSR_WRITE) > > + return handle_wrmsr(vcpu); > > + else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER) > > + return handle_preemption_timer(vcpu); > > + else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT) > > + return handle_interrupt_window(vcpu); > > + else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT) > > + return handle_external_interrupt(vcpu); > > + else if (exit_reason == EXIT_REASON_HLT) > > + return handle_halt(vcpu); > > + else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION) > > + return handle_pause(vcpu); > > + else if (exit_reason == EXIT_REASON_MSR_READ) > > + return handle_rdmsr(vcpu); > > + else if (exit_reason == EXIT_REASON_CPUID) > > + return handle_cpuid(vcpu); > > + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG) > > + return handle_ept_misconfig(vcpu); > > +#endif > > return kvm_vmx_exit_handlers[exit_reason](vcpu); > > Most of these, while frequent, are already part of slow paths. > > I would keep only EXIT_REASON_MSR_WRITE, EXIT_REASON_PREEMPTION_TIMER, > EXIT_REASON_EPT_MISCONFIG and add EXIT_REASON_IO_INSTRUCTION. Intuition doesn't work great when it comes to CPU speculative execution runtime. I can however run additional benchmarks to verify your theory that keeping around frequent retpolines will still perform ok. > If you make kvm_vmx_exit_handlers const, can the compiler substitute for > instance kvm_vmx_exit_handlers[EXIT_REASON_MSR_WRITE] with handle_wrmsr? > Just thinking out loud, not sure if it's an improvement code-wise. gcc gets right if you make it const, it calls kvm_emulate_wrmsr in fact. However I don't think const will fly with_vmx_hardware_setup()... in fact at runtime testing nested I just got: BUG: unable to handle page fault for address: ffffffffa00751e0 #PF: supervisor write access in kernel mode #PF: error_code(0x0003) - permissions violation PGD 2424067 P4D 2424067 PUD 2425063 PMD 7cc09067 PTE 80000000741cb161 Oops: 0003 [#1] SMP NOPTI CPU: 1 PID: 4458 Comm: insmod Not tainted 5.3.0+ #1 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS ?-20190711_202441-buildvm-armv7-10.arm.fedoraproject.or4 RIP: 0010:nested_vmx_hardware_setup+0x29a/0x37a [kvm_intel] Code: 41 ff c5 66 89 2c 85 20 92 0b a0 66 44 89 34 85 22 92 0b a0 49 ff c7 e9 e6 fe ff ff 44 89 2d 28 24 fc ff 48 RSP: 0018:ffffc90000257c18 EFLAGS: 00010246 RAX: ffffffffa001e0b0 RBX: ffffffffa0075140 RCX: 0000000000000000 RDX: ffff888078f60000 RSI: 0000000000002401 RDI: 0000000000000018 RBP: 0000000000006c08 R08: 0000000000001000 R09: 000000000007ffdc R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000006c08 R13: 0000000000000017 R14: 0000000000000268 R15: 0000000000000018 FS: 00007f7fb7ef0b80(0000) GS:ffff88807da40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: ffffffffa00751e0 CR3: 0000000079620001 CR4: 0000000000160ee0 Call Trace: hardware_setup+0x4df/0x5b2 [kvm_intel] kvm_arch_hardware_setup+0x2f/0x27b [kvm_intel] kvm_init+0x5d/0x26d [kvm_intel]
