Messages in this thread |  | | Subject | Re: [PATCH 2/3] KVM: X86: implement the logic for spinlock optimization | From | "Longpeng(Mike)" <> | Date | Mon, 7 Aug 2017 20:28:13 +0800 |
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On 08/07/2017 06:45 PM, Paolo Bonzini wrote: > On 07/08/2017 10:44, Longpeng(Mike) wrote: >> + >> + /* >> + * Intel sdm vol3 ch-25.1.3 says: The “PAUSE-loop exiting” >> + * VM-execution control is ignored if CPL > 0. So the vcpu >> + * is always exiting with CPL=0 if it uses PLE. > > This is not true (how can it be?). What 25.1.3 says is, the VCPU is > always at CPL=0 if you get a PAUSE exit (reason 40) and PAUSE exiting is > 0 (it always is for KVM). But here you're looking for a VCPU that > didn't get a PAUSE exit, so the CPL can certainly be 3. >
Hi Paolo,
My comment above is something wrong(please forgive my poor English), my origin meaning is: The “PAUSE-loop exiting” VM-execution control is ignored if CPL > 0. So the vcpu's CPL is must 0 if it exits due to PLE.
* kvm_arch_spin_in_kernel() returns whether the vcpu(which exits due to spinlock) is CPL=0. It only be called by kvm_vcpu_on_spin(), and the input vcpu is 'me' which get a PAUSE exit now. *
I split kvm_arch_vcpu_in_kernel(in RFC) into two functions: kvm_arch_spin_in_kernel and kvm_arch_preempt_in_kernel
Because of KVM/VMX L1 never set CPU_BASED_PAUSE_EXITING and only set SECONDARY_EXEC_PAUSE_LOOP_EXITING if supported, so for L1: 1. get a PAUSE exit with CPL=0 if PLE is supported 2. never get a PAUSE exit if don't support PLE
So, I think it can direct return true(CPL=0) if supports PLE.
But for nested KVM/VMX(I'm not familiar with nested), it could set CPU_BASED_PAUSE_EXITING, so I think get_cpl() is also needed.
If the above is correct, what about this way( we can save a vmcs_read opeartion for L1):
kvm_arch_vcpu_spin_in_kernel(vcpu) { if (!is_guest_mode(vcpu)) return true;
return vmx_get_cpl(vcpu) == 0; }
kvm_vcpu_on_spin() { /* @me get a PAUSE exit */ me_in_kernel = kvm_arch_vcpu_spin_in_kernel(me); ... for each vcpu { ... if (me_in_kernel && !...preempt_in_kernel(vcpu)) continue; ... } ... }
--- Regards, Longpeng(Mike)
> However, I understand that vmx_get_cpl can be a bit slow here. You can > actually read SS's access rights directly in this function and get the > DPL from there, that's going to be just a single VMREAD. > > The only difference is when vmx->rmode.vm86_active=1. However, > pause-loop exiting is not working properly anyway if > vmx->rmode.vm86_active=1, because CPL=3 according to the processor. > > Paolo > >> + * The following block needs less cycles than vmx_get_cpl(). >> + */ >> + if (cpu_has_secondary_exec_ctrls()) >> + secondary_exec_ctrl = vmcs_read32(SECONDARY_VM_EXEC_CONTROL); >> + if (secondary_exec_ctrl & SECONDARY_EXEC_PAUSE_LOOP_EXITING) >> + return true; >> + > > Paolo >
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