linux-kernel - Re: [PATCH v3 2/3] KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode

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Date:   Thu, 5 Oct 2017 14:40:58 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v3 2/3] KVM: LAPIC: Keep timer running when switching
 between one-shot and periodic mode

2017-10-05 03:53-0700, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> If we take TSC-deadline mode timer out of the picture, the Intel SDM
> does not say that the timer is disable when the timer mode is change,
> either from one-shot to periodic or vice versa.
> 
> After this patch, the timer is no longer disarmed on change of mode, so
> the counter (TMCCT) keeps counting down.
> 
> So what does a write to LVTT changes ? On baremetal, the change of mode
> is probably taken into account only when the counter reach 0. When this
> happen, LVTT is use to figure out if the counter should restard counting
> down from TMICT (so periodic mode) or stop counting (if one-shot mode).
> 
> This patch is based on observation of the behavior of the APIC timer on
> baremetal as well as check that they does not go against the description
> written in the Intel SDM.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/lapic.c | 45 +++++++++++++++++++++++++++++++--------------
>  1 file changed, 31 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -1729,7 +1745,8 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>  			val |= APIC_LVT_MASKED;
>  		val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
>  		kvm_lapic_set_reg(apic, APIC_LVTT, val);
> -		apic_update_lvtt(apic);
> +		if (apic_update_lvtt(apic) && !apic_lvtt_tscdeadline(apic))
> +			start_apic_timer(apic, true);

start_apic_timer() is not needed here: when switching from
apic_lvtt_tscdeadline(), we have TMICT = 0, so the timer is disabled.
When switching between one-shot and periodic, the timer is running with
a correct expiration time.

This will bring us close to bare-metal behavior wrt. races and also
allows us to get rid of apic_update_lvtt() return value and an argument
to start_apic_timer().

Thanks.