Re: [Xen-devel] [PATCH] xen: Don't disable the REP emulation optimizations for regular IO

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.03.16 at 11:07, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> Currently REP emulations optimizations remain disabled even if
> the emulation does not happen as a result of a vm_event reply
> requestion emulation (i.e. even for regular IO). This patch takes
> emulate_each_rep into account only if emulation has been requested
> by a vm_event-capable application, and is a noticeable speed
> optimization for monitored guests.
> 
> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>

Looks reasonable, but this being an event specific change I'm
making my ack to this dependent on Tamas'es consent.

Jan

> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -498,6 +498,7 @@ static int hvmemul_virtual_to_linear(
>  {
>      struct segment_register *reg;
>      int okay;
> +    unsigned long max_reps = 4096;
>  
>      if ( seg == x86_seg_none )
>      {
> @@ -506,16 +507,21 @@ static int hvmemul_virtual_to_linear(
>      }
>  
>      /*
> +     * If introspection has been enabled for this domain, and we're emulating
> +     * becase a vm_reply asked us to (i.e. not doing regular IO) reps should
> +     * be at most 1, since optimization might otherwise cause a single
> +     * vm_event being triggered for repeated writes to a whole page.
> +     */
> +    if ( unlikely(current->domain->arch.mem_access_emulate_each_rep) &&
> +         current->arch.vm_event->emulate_flags != 0 )
> +       max_reps = 1;
> +
> +    /*
>       * Clip repetitions to avoid overflow when multiplying by @bytes_per_rep.
>       * The chosen maximum is very conservative but it's what we use in
>       * hvmemul_linear_to_phys() so there is no point in using a larger value.
> -     * If introspection has been enabled for this domain, *reps should be
> -     * at most 1, since optimization might otherwise cause a single vm_event
> -     * being triggered for repeated writes to a whole page.
>       */
> -    *reps = min_t(unsigned long, *reps,
> -                  unlikely(current->domain->arch.mem_access_emulate_each_rep)
> -                           ? 1 : 4096);
> +    *reps = min_t(unsigned long, *reps, max_reps);
>  
>      reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
>  
> -- 
> 1.9.1




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