Re: [PATCH v6 4/5] VT-d: Reduce spin timeout to 1ms, which can be boot-time changed

[Dario Faggioli <dario.faggioli@citrix.com>]

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[Trimmed the Cc-list a bit again]

On Wed, 2016-03-02 at 22:31 +0800, Quan Xu wrote:
> Signed-off-by: Quan Xu <quan.xu@intel.com>
>  
> diff --git a/xen/drivers/passthrough/vtd/qinval.c
> b/xen/drivers/passthrough/vtd/qinval.c
> index b81b0bd..882b9f4 100644
> --- a/xen/drivers/passthrough/vtd/qinval.c
> +++ b/xen/drivers/passthrough/vtd/qinval.c
> @@ -28,6 +28,11 @@
>  #include "vtd.h"
>  #include "extern.h"
>  
> +static unsigned int __read_mostly vtd_qi_timeout = 1;
> +integer_param("vtd_qi_timeout", vtd_qi_timeout);
> +
> +#define IOMMU_QI_TIMEOUT (vtd_qi_timeout * MILLISECS(1))
> +
>  static void print_qi_regs(struct iommu *iommu)
>  {
>      u64 val;
> @@ -130,6 +135,10 @@ static void queue_invalidate_iotlb(struct iommu
> *iommu,
>      spin_unlock_irqrestore(&iommu->register_lock, flags);
>  }
>  
> +/*
> + * NB. We must check all kinds of error and all the way up the
> + * call trees.
> + */
>  static int queue_invalidate_wait(struct iommu *iommu,
>      u8 iflag, u8 sw, u8 fn)
>  {
> @@ -167,10 +176,12 @@ static int queue_invalidate_wait(struct iommu
> *iommu,
>          start_time = NOW();
>          while ( poll_slot != QINVAL_STAT_DONE )
>          {
> -            if ( NOW() > (start_time + DMAR_OPERATION_TIMEOUT) )
> +            if ( NOW() > (start_time + IOMMU_QI_TIMEOUT) )
>
Since this now involves a time unit conversion, can't we:
 - instead of start_time, above, compute, once and for all:
     timeout = NOW() + IOMMU_QI_TIMEOUT;
 - check whether ( NOW() > timeout )

I appreciate that the default for vtd_qi_timeout is 1, so it's most
likely not that a big deal, but it still looks better to me.

>              {
>                  print_qi_regs(iommu);
> -                panic("queue invalidate wait descriptor was not
> executed");
> +                printk(XENLOG_WARNING VTDPREFIX
> +                       "Queue invalidate wait descriptor was
> timeout.\n");
>
"Queue invalidate wait descriptor timed out"  ?

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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