Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753177AbdLERKF (ORCPT ); Tue, 5 Dec 2017 12:10:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46848 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753100AbdLERKB (ORCPT ); Tue, 5 Dec 2017 12:10:01 -0500 Date: Tue, 5 Dec 2017 18:09:55 +0100 From: Radim Krcmar To: Wanpeng Li Cc: Rik van Riel , Paolo Bonzini , kvm , "linux-kernel@vger.kernel.org" , Thomas Gleixner , Christian Borntraeger Subject: Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run Message-ID: <20171205170951.GA829@flask> References: <20171114001223.441ea2ca@annuminas.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 05 Dec 2017 17:10:01 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5766 Lines: 113 2017-12-04 10:15+0800, Wanpeng Li: > 2017-11-14 13:12 GMT+08:00 Rik van Riel : > > Currently, every time a VCPU is scheduled out, the host kernel will > > first save the guest FPU/xstate context, then load the qemu userspace > > FPU context, only to then immediately save the qemu userspace FPU > > context back to memory. When scheduling in a VCPU, the same extraneous > > FPU loads and saves are done. > > > > This could be avoided by moving from a model where the guest FPU is > > loaded and stored with preemption disabled, to a model where the > > qemu userspace FPU is swapped out for the guest FPU context for > > the duration of the KVM_RUN ioctl. > > > > This is done under the VCPU mutex, which is also taken when other > > tasks inspect the VCPU FPU context, so the code should already be > > safe for this change. That should come as no surprise, given that > > s390 already has this optimization. > > > > No performance changes were detected in quick ping-pong tests on > > my 4 socket system, which is expected since an FPU+xstate load is > > on the order of 0.1us, while ping-ponging between CPUs is on the > > order of 20us, and somewhat noisy. > > > > There may be other tests where performance changes are noticeable. > > The kvm/queue has the below splatting: > > [ 650.866212] Bad FPU state detected at kvm_put_guest_fpu+0x7d/0x210 > [kvm], reinitializing FPU registers. > [ 650.866232] ------------[ cut here ]------------ > [ 650.866241] WARNING: CPU: 2 PID: 2583 at arch/x86/mm/extable.c:103 > ex_handler_fprestore+0x5f/0x70 > [ 650.866473] libahci wmi hid pinctrl_sunrisepoint video pinctrl_intel > [ 650.866496] CPU: 2 PID: 2583 Comm: qemu-system-x86 Not tainted 4.14.0+ #7 > [ 650.866500] Hardware name: Dell Inc. OptiPlex 7040/0JCTF8, BIOS > 1.4.9 09/12/2016 > [ 650.866503] task: ffff97a095a28000 task.stack: ffffa71c8585c000 > [ 650.866509] RIP: 0010:ex_handler_fprestore+0x5f/0x70 > [ 650.866512] RSP: 0018:ffffa71c8585fc28 EFLAGS: 00010282 > [ 650.866519] RAX: 000000000000005b RBX: ffffa71c8585fc68 RCX: 0000000000000006 > [ 650.866522] RDX: 0000000000000000 RSI: ffffffffb4d35333 RDI: 0000000000000282 > [ 650.866526] RBP: 000000000000000d R08: 00000000fddae359 R09: 0000000000000000 > [ 650.866529] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 > [ 650.866532] R13: 0000000000000000 R14: ffff97a095a30000 R15: 000055824b58e280 > [ 650.866536] FS: 00007f6f8f22c700(0000) GS:ffff97a09ca00000(0000) > knlGS:0000000000000000 > [ 650.866540] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 650.866543] CR2: 00007f6f993f3000 CR3: 00000003d4aae005 CR4: 00000000003626e0 > [ 650.866547] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > [ 650.866550] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > [ 650.866554] Call Trace: > [ 650.866559] fixup_exception+0x32/0x40 > [ 650.866567] do_general_protection+0xa0/0x1b0 > [ 650.866574] general_protection+0x22/0x30 > [ 650.866595] RIP: 0010:kvm_put_guest_fpu+0x7d/0x210 [kvm] > [ 650.866599] RSP: 0018:ffffa71c8585fd18 EFLAGS: 00010246 > [ 650.866605] RAX: 00000000ffffffff RBX: ffff97a095a30000 RCX: 0000000000000001 > [ 650.866608] RDX: 00000000ffffffff RSI: 00000000f7d5d46a RDI: ffff97a095a30b80 > [ 650.866611] RBP: 0000000000000000 R08: 00000000fddae359 R09: ffff97a095a28968 > [ 650.866615] R10: 0000000000000000 R11: 00000000e8d39b88 R12: ffff97a095a31bc0 > [ 650.866618] R13: 0000000000000000 R14: ffff97a095a30000 R15: 000055824b58e280 > [ 650.866650] ? kvm_put_guest_fpu+0x27/0x210 [kvm] Looks like we're calling put when the fpu was not loaded. The simplest fix would be: diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index e0367f688547..064eba25c215 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -7842,7 +7842,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) * To avoid have the INIT path from kvm_apic_has_events() that be * called with loaded FPU and does not let userspace fix the state. */ - kvm_put_guest_fpu(vcpu); + if (init_event) + kvm_put_guest_fpu(vcpu); mpx_state_buffer = get_xsave_addr(&vcpu->arch.guest_fpu.state.xsave, XFEATURE_MASK_BNDREGS); if (mpx_state_buffer) @@ -7851,6 +7852,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) XFEATURE_MASK_BNDCSR); if (mpx_state_buffer) memset(mpx_state_buffer, 0, sizeof(struct mpx_bndcsr)); + if (init_event) + kvm_load_guest_fpu(vcpu); } if (!init_event) { I'll carry that until there is a nicer solution, thanks for the report. > [ 650.866670] kvm_vcpu_reset+0x1be/0x250 [kvm] > [ 650.866689] kvm_arch_vcpu_setup+0x2c/0x50 [kvm] > [ 650.866707] kvm_vm_ioctl+0x31a/0x820 [kvm] > [ 650.866712] ? __lock_acquire+0x809/0x1410 > [ 650.866721] ? __lock_acquire+0x809/0x1410 > [ 650.866734] do_vfs_ioctl+0x9f/0x6c0 > [ 650.866743] ? __fget+0x108/0x1f0 > [ 650.866752] SyS_ioctl+0x74/0x80 > [ 650.866757] ? do_syscall_64+0xc4/0x3d0 > [ 650.866764] do_syscall_64+0x8a/0x3d0 > [ 650.866769] ? trace_hardirqs_on_thunk+0x1a/0x1c > [ 650.866781] entry_SYSCALL64_slow_path+0x25/0x25 > [ 650.866785] RIP: 0033:0x7f6f973a0f07 > [ 650.866788] RSP: 002b:00007f6f8f22b968 EFLAGS: 00000246 ORIG_RAX: > 0000000000000010 > [ 650.866795] RAX: ffffffffffffffda RBX: 000000000000ae41 RCX: 00007f6f973a0f07 > [ 650.866798] RDX: 0000000000000000 RSI: 000000000000ae41 RDI: 000000000000000b > [ 650.866802] RBP: 0000000000000000 R08: 0000558248e26a40 R09: 000055824b58e280 > [ 650.866805] R10: 0000558249593f40 R11: 0000000000000246 R12: 000055824b55ec90 > [ 650.866808] R13: 00007ffd274d79ff R14: 00007f6f8f22c9c0 R15: 000055824b58e280 > [ 650.867014] ---[ end trace 2c5d6cfaba0ee1b3 ]---