linux-kernel - Re: [PATCH v4 2/2] KVM: nVMX: Fix preemption timer bit set in vmcs02 even if L1 doesn't enable it

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Date:	Thu, 7 Jul 2016 16:11:47 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Haozhong Zhang <haozhong.zhang@...el.com>
Subject: Re: [PATCH v4 2/2] KVM: nVMX: Fix preemption timer bit set in vmcs02
 even if L1 doesn't enable it



On 07/07/2016 15:23, Wanpeng Li wrote:
>>> >>
>>> >>               if (kvm_lapic_hv_timer_in_use(vcpu) &&
>>> >> +                     (is_guest_mode(vcpu) ||
>>> >>                               kvm_x86_ops->set_hv_timer(vcpu,
>>> >> -                                     kvm_get_lapic_tscdeadline_msr(vcpu)))
>>> >> +                                     kvm_get_lapic_tscdeadline_msr(vcpu))))
>>> >>                       kvm_lapic_switch_to_sw_timer(vcpu);
>>> >>               if (check_tsc_unstable()) {
>>> >>                       u64 offset = kvm_compute_tsc_offset(vcpu,
>>> >>
>> >
>> > Thanks, this is good as a fallback.  I'll try to fix it by getting the
>> > pin-based execution controls right but if I fail this patch is okay.
> I believe we still need this patch even if you implement "L1 TSC
> deadline timer to trigger while L2 is running" eventually, the codes
> you posted before:
> 
>   exec_control = vmcs12->pin_based_vm_exec_control;
> +exec_control &= ~PIN_BASED_VMX_PREEMPTION_TIMER;
>   exec_control |= vmcs_config.pin_based_exec_ctrl;
> - exec_control &= ~PIN_BASED_VMX_PREEMPTION_TIMER;
> + if (vmx->hv_deadline_tsc == -1)
> +     exec_control &= ~PIN_BASED_VMX_PREEMPTION_TIMER;
> 
> So there is still case the preemption timer bit of vmcs02 is not set,
> however,  the scenario I mentioned above in kvm_arch_vcpu_load() will
> set it unnecessary.

kvm_x86_ops->set_hv_timer _will_ set the preemption timer bit of vmcs02
if vmcs02 is the loaded one.

This can happen if L2 has access to L1's local APIC registers (i.e. L1
passes the local APIC instead of emulating it, as is the case in a
partitioning hypervisor).  While L2 runs, it writes to the TSC deadline
MSR of L1.  This causes a call to kvm_x86_ops->set_hv_timer while the
active VMCS is a vmcs02.

Paolo