linux-kernel - Re: [External] Re: [RFC: timer passthrough 1/9] KVM: vmx: hook set_next

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Date:   Tue, 23 Feb 2021 21:07:48 +0800
From:   Zhimin Feng <fengzhimin@...edance.com>
To:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, fweisbec@...il.com,
        zhouyibo@...edance.com, zhanghaozhong@...edance.com
Subject: Re: [External] Re: [RFC: timer passthrough 1/9] KVM: vmx: hook
 set_next_event for getting the host tscd

Hi tglx

This question is very nice,  we should be considered to judge whether 
the current active device is the tsc deadline timer. I will fix this in V2.

Thanks

Zhimin

在 2021/2/6 上午2:11, Thomas Gleixner 写道:
> On Fri, Feb 05 2021 at 18:03, Zhimin Feng wrote:
>> @@ -520,6 +521,24 @@ struct kvm_vcpu_hv {
>>   	cpumask_t tlb_flush;
>>   };
>>   
>> +enum tick_device_mode {
>> +	TICKDEV_MODE_PERIODIC,
>> +	TICKDEV_MODE_ONESHOT,
>> +};
>> +
>> +struct tick_device {
>> +	struct clock_event_device *evtdev;
>> +	enum tick_device_mode mode;
>> +};
> There is a reason why these things are defined in a header file which is
> not public. Nothing outside of kernel/time/ has to fiddle with
> this. Aside of that how are these things supposed to stay in sync?
>
>> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
>> index 6c9c342dd0e5..bc50f4a1a7c0 100644
>> --- a/kernel/time/tick-common.c
>> +++ b/kernel/time/tick-common.c
>> @@ -26,6 +26,7 @@
>>    * Tick devices
>>    */
>>   DEFINE_PER_CPU(struct tick_device, tick_cpu_device);
>> +EXPORT_SYMBOL_GPL(tick_cpu_device);
> Not going to happen ever.
>
>> +#define TSC_DIVISOR  8
>> +static DEFINE_PER_CPU(struct timer_passth_info, passth_info);
>> +
>> +static int override_lapic_next_event(unsigned long delta,
>> +		struct clock_event_device *evt)
>> +{
>> +	struct timer_passth_info *local_timer_info;
>> +	u64 tsc;
>> +	u64 tscd;
>> +
>> +	local_timer_info = &per_cpu(passth_info, smp_processor_id());
>> +	tsc = rdtsc();
>> +	tscd = tsc + (((u64) delta) * TSC_DIVISOR);
>> +	local_timer_info->host_tscd = tscd;
>> +	wrmsrl(MSR_IA32_TSCDEADLINE, tscd);
>> +	return 0;
>> +}
>> +
>> +static void vmx_host_timer_passth_init(void *junk)
>> +{
>> +	struct timer_passth_info *local_timer_info;
>> +	int cpu = smp_processor_id();
>> +
>> +	local_timer_info = &per_cpu(passth_info, cpu);
>> +	local_timer_info->curr_dev = per_cpu(tick_cpu_device, cpu).evtdev;
>> +	local_timer_info->orig_set_next_event =
>> +		local_timer_info->curr_dev->set_next_event;
>> +	local_timer_info->curr_dev->set_next_event = override_lapic_next_event;
> So when loading the KVM module you steal the set_next_event pointer of
> the clock event device which is currently active. What guarantees that
>
>      1) The current active device is the tsc deadline timer
>      2) The active device does not change
>
> Nothing.
>
> Thanks,
>
>          tglx