Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run

From: Wanpeng Li
Date: Sun Dec 03 2017 - 21:16:02 EST

2017-11-14 13:12 GMT+08:00 Rik van Riel <riel@xxxxxxxxxx>:
> 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]
[ 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 ]---

Regards,
Wanpeng Li

>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> Suggested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 13 +++++++++++++
> arch/x86/kvm/x86.c | 29 ++++++++++++-----------------
> 2 files changed, 25 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c73e493adf07..92e66685249e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -536,7 +536,20 @@ struct kvm_vcpu_arch {
> struct kvm_mmu_memory_cache mmu_page_cache;
> struct kvm_mmu_memory_cache mmu_page_header_cache;
>
> + /*
> + * QEMU userspace and the guest each have their own FPU state.
> + * In vcpu_run, we switch between the user and guest FPU contexts.
> + * While running a VCPU, the VCPU thread will have the guest FPU
> + * context.
> + *
> + * Note that while the PKRU state lives inside the fpu registers,
> + * it is switched out separately at VMENTER and VMEXIT time. The
> + * "guest_fpu" state here contains the guest FPU context, with the
> + * host PRKU bits.
> + */
> + struct fpu user_fpu;
> struct fpu guest_fpu;
> +
> u64 xcr0;
> u64 guest_supported_xcr0;
> u32 guest_xstate_size;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb7fcd6..59912b20a830 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2917,7 +2917,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> srcu_read_unlock(&vcpu->kvm->srcu, idx);
> pagefault_enable();
> kvm_x86_ops->vcpu_put(vcpu);
> - kvm_put_guest_fpu(vcpu);
> vcpu->arch.last_host_tsc = rdtsc();
> }
>
> @@ -6908,7 +6907,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> preempt_disable();
>
> kvm_x86_ops->prepare_guest_switch(vcpu);
> - kvm_load_guest_fpu(vcpu);
>
> /*
> * Disable IRQs before setting IN_GUEST_MODE. Posted interrupt
> @@ -7095,6 +7093,8 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>
> vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
>
> + kvm_load_guest_fpu(vcpu);
> +
> for (;;) {
> if (kvm_vcpu_running(vcpu)) {
> r = vcpu_enter_guest(vcpu);
> @@ -7132,6 +7132,8 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
> }
> }
>
> + kvm_put_guest_fpu(vcpu);
> +
> srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
>
> return r;
> @@ -7663,32 +7665,25 @@ static void fx_init(struct kvm_vcpu *vcpu)
> vcpu->arch.cr0 |= X86_CR0_ET;
> }
>
> +/* Swap (qemu) user FPU context for the guest FPU context. */
> void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - if (vcpu->guest_fpu_loaded)
> - return;
> -
> - /*
> - * Restore all possible states in the guest,
> - * and assume host would use all available bits.
> - * Guest xcr0 would be loaded later.
> - */
> - vcpu->guest_fpu_loaded = 1;
> - __kernel_fpu_begin();
> + preempt_disable();
> + copy_fpregs_to_fpstate(&vcpu->arch.user_fpu);
> /* PKRU is separately restored in kvm_x86_ops->run. */
> __copy_kernel_to_fpregs(&vcpu->arch.guest_fpu.state,
> ~XFEATURE_MASK_PKRU);
> + preempt_enable();
> trace_kvm_fpu(1);
> }
>
> +/* When vcpu_run ends, restore user space FPU context. */
> void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - if (!vcpu->guest_fpu_loaded)
> - return;
> -
> - vcpu->guest_fpu_loaded = 0;
> + preempt_disable();
> copy_fpregs_to_fpstate(&vcpu->arch.guest_fpu);
> - __kernel_fpu_end();
> + copy_kernel_to_fpregs(&vcpu->arch.user_fpu.state);
> + preempt_enable();
> ++vcpu->stat.fpu_reload;
> trace_kvm_fpu(0);
> }
>