On Tue, 2007-10-23 at 16:19 +0200, Avi Kivity wrote: > Jan Kiszka wrote: > > Avi, > > > > [somehow your mails do not get through to my private account, so I'm > > switching] > > > > Avi Kivity wrote: > > > >> Jan Kiszka wrote: > >> > >>> Clarification: I can't precisely tell what code is executed in VM mode, > >>> as I don't have qemu or that guest instrumented. I just see the kernel > >>> entering VM mode and leaving it again more than 300 us later. So I > >>> wonder why this is allowed while some external IRQ is pending. > >>> > >>> > >>> > >> How do you know an external interrupt is pending? > >> > > > > It's the host timer IRQ, programmed to fire in certain intervals (100 us > > here). Test case is some latency measurement tool like tglx's cyclictest > > or similar programs we use in Xenomai. > > > > > >> kvm programs the hardware to exit when an external interrupt arrives. > >> > >> > > > > Here is a latency trace I just managed to capture over 2.6.23.1-rt1 with > > latest kvm from git hacked into (kvm generally seems to work fine this way): > > > > ... > > qemu-sys-7543 0...1 13897us : vmcs_write16+0xb/0x20 > > (vmx_save_host_state+0x1a7/0x1c0) > > qemu-sys-7543 0...1 13897us : vmcs_writel+0xb/0x30 (vmcs_write16+0x1e/0x20) > > qemu-sys-7543 0...1 13898us : segment_base+0xc/0x70 > > (vmx_save_host_state+0xa0/0x1c0) > > qemu-sys-7543 0...1 13898us : vmcs_writel+0xb/0x30 > > (vmx_save_host_state+0xb0/0x1c0) > > qemu-sys-7543 0...1 13898us : segment_base+0xc/0x70 > > (vmx_save_host_state+0xbf/0x1c0) > > qemu-sys-7543 0...1 13898us : vmcs_writel+0xb/0x30 > > (vmx_save_host_state+0xcf/0x1c0) > > qemu-sys-7543 0...1 13898us : load_msrs+0xb/0x40 > > (vmx_save_host_state+0xe7/0x1c0) > > qemu-sys-7543 0...1 13898us : kvm_load_guest_fpu+0x8/0x40 > > (kvm_vcpu_ioctl_run+0xbf/0x570) > > qemu-sys-7543 0D..1 13899us : vmx_vcpu_run+0xc/0x110 > > (kvm_vcpu_ioctl_run+0x120/0x570) > > qemu-sys-7543 0D..1 13899us!: vmcs_writel+0xb/0x30 > > (vmx_vcpu_run+0x22/0x110) > > qemu-sys-7543 0D..1 14344us : vmcs_read32+0xb/0x20 > > (vmx_vcpu_run+0xc7/0x110) > > qemu-sys-7543 0D..1 14345us : vmcs_readl+0x8/0x10 (vmcs_read32+0x16/0x20) > > qemu-sys-7543 0D..1 14345us : vmcs_read32+0xb/0x20 > > (vmx_vcpu_run+0xf4/0x110) > > qemu-sys-7543 0D..1 14345us+: vmcs_readl+0x8/0x10 (vmcs_read32+0x16/0x20) > > qemu-sys-7543 0D..1 14349us : irq_enter+0xb/0x30 (do_IRQ+0x45/0xc0) > > qemu-sys-7543 0D.h1 14350us : do_IRQ+0x73/0xc0 (f8caae24 0 0) > > qemu-sys-7543 0D.h1 14351us : handle_level_irq+0xe/0x120 (do_IRQ+0x7d/0xc0) > > qemu-sys-7543 0D.h1 14351us : __spin_lock+0xc/0x30 > > (handle_level_irq+0x24/0x120) > > qemu-sys-7543 0D.h2 14352us : mask_and_ack_8259A+0x14/0x120 > > (handle_level_irq+0x37/0x120) > > qemu-sys-7543 0D.h2 14352us+: __spin_lock_irqsave+0x11/0x60 > > (mask_and_ack_8259A+0x2a/0x120) > > qemu-sys-7543 0D.h3 14357us : __spin_unlock_irqrestore+0xc/0x60 > > (mask_and_ack_8259A+0x7a/0x120) > > qemu-sys-7543 0D.h2 14358us : redirect_hardirq+0x8/0x70 > > (handle_level_irq+0x72/0x120) > > qemu-sys-7543 0D.h2 14358us : __spin_unlock+0xb/0x40 > > (handle_level_irq+0x8e/0x120) > > qemu-sys-7543 0D.h1 14358us : handle_IRQ_event+0xe/0x110 > > (handle_level_irq+0x9a/0x120) > > qemu-sys-7543 0D.h1 14359us : timer_interrupt+0xb/0x60 > > (handle_IRQ_event+0x67/0x110) > > qemu-sys-7543 0D.h1 14359us : hrtimer_interrupt+0xe/0x1f0 > > (timer_interrupt+0x20/0x60) > > ... > > > > One can see 345 us latency between vm-enter and vm-exit in vmx_vcpu_run - > > and this while cyclictest runs at a period of 100 us! > > > > I got the same results over Adeos/I-pipe & Xenomai with the function > > tracer there, also pointing to the period while the CPU is in VM mode. > > > > Anyone any ideas? Greg, I put you on CC as you said you once saw "decent > > latencies" with your patches. Are there still magic bits missing in > > official kvm? > > > > No bits missing as far as I know. It should just work.
That could very well be the case these days. I know back when I was looking at it, KVM would not run on VMX + -rt without modification or it would crash/hang (this was around the time I was working on that smp_function_call stuff). And without careful modification it would run very poorly (with high (300us+ latencies) revealed in cyclictest. However, I was able to craft the vmx_vcpu_run path so that a VM could run side-by-side with cyclictest with sub 40us latencies. In fact, normally it was sub 30us, but on an occasional run I would get a spike to ~37us. Unfortunately I am deep into other non-KVM related -rt issues at the moment, so I can't work on it any further for a bit. Regards, -Greg
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