Re: KVM: x86: limit PIT timer frequency

[Jan Kiszka <jan.kiszka@web.de>]

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On 2014-01-06 15:00, Marcelo Tosatti wrote:
> 
> 
> Limit PIT timer frequency similarly to the limit applied by 
> LAPIC timer.
> 
> Cc: stable@kernel.org
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index 412a5aa..518d864 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -37,6 +37,7 @@
>  
>  #include "irq.h"
>  #include "i8254.h"
> +#include "x86.h"
>  
>  #ifndef CONFIG_X86_64
>  #define mod_64(x, y) ((x) - (y) * div64_u64(x, y))
> @@ -349,6 +350,23 @@ static void create_pit_timer(struct kvm *kvm, u32 val, int is_period)
>  	atomic_set(&ps->pending, 0);
>  	ps->irq_ack = 1;
>  
> +	/*
> +	 * Do not allow the guest to program periodic timers with small
> +	 * interval, since the hrtimers are not throttled by the host
> +	 * scheduler.
> +	 */
> +	if (ps->is_periodic) {
> +		s64 min_period = min_timer_period_us * 1000LL;
> +
> +		if (ps->period < min_period) {
> +			pr_info_ratelimited(
> +			    "kvm: requested %lld ns "
> +			    "i8254 timer period limited to %lld ns\n",
> +			    ps->period, min_period);
> +			ps->period = min_period;
> +		}
> +	}
> +
>  	hrtimer_start(&ps->timer, ktime_add_ns(ktime_get(), interval),
>  		      HRTIMER_MODE_ABS);
>  }
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index dec48bf..7f4dfa6 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -71,9 +71,6 @@
>  #define VEC_POS(v) ((v) & (32 - 1))
>  #define REG_POS(v) (((v) >> 5) << 4)
>  
> -static unsigned int min_timer_period_us = 500;
> -module_param(min_timer_period_us, uint, S_IRUGO | S_IWUSR);
> -
>  static inline void apic_set_reg(struct kvm_lapic *apic, int reg_off, u32 val)
>  {
>  	*((u32 *) (apic->regs + reg_off)) = val;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5d004da..d89d51b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -94,6 +94,9 @@ EXPORT_SYMBOL_GPL(kvm_x86_ops);
>  static bool ignore_msrs = 0;
>  module_param(ignore_msrs, bool, S_IRUGO | S_IWUSR);
>  
> +unsigned int min_timer_period_us = 500;
> +module_param(min_timer_period_us, uint, S_IRUGO | S_IWUSR);
> +
>  bool kvm_has_tsc_control;
>  EXPORT_SYMBOL_GPL(kvm_has_tsc_control);
>  u32  kvm_max_guest_tsc_khz;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 587fb9e..8da5823 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -125,5 +125,7 @@ int kvm_write_guest_virt_system(struct x86_emulate_ctxt *ctxt,
>  #define KVM_SUPPORTED_XCR0	(XSTATE_FP | XSTATE_SSE | XSTATE_YMM)
>  extern u64 host_xcr0;
>  
> +extern unsigned int min_timer_period_us;
> +
>  extern struct static_key kvm_no_apic_vcpu;
>  #endif
> 

Indeed, required here as well.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>

At this chance, I also grep'ed through timer emulations of other archs.
Only MIPS uses a periodic hrtimer, but with a fixed frequency. So we
should be safe for now.

Jan


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