Re: [Xen-devel] [PATCH-4.5 3/4] xen/arm: do not request maintenance_interrupts

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-02-10 at 17:24 +0000, Stefano Stabellini wrote:
> On Mon, 10 Feb 2014, Ian Campbell wrote:
> > On Mon, 2014-02-10 at 17:16 +0000, Stefano Stabellini wrote:
> > > On Mon, 10 Feb 2014, Ian Campbell wrote:
> > > > On Mon, 2014-02-10 at 17:06 +0000, Stefano Stabellini wrote:
> > > > > On Fri, 7 Feb 2014, Julien Grall wrote:
> > > > > > On 07/02/14 18:56, Stefano Stabellini wrote:
> > > > > >    > +static void gic_clear_lrs(struct vcpu *v)
> > > > > > > +{
> > > > > > > +    struct pending_irq *p;
> > > > > > > +    int i = 0, irq;
> > > > > > > +    uint32_t lr;
> > > > > > > +    bool_t inflight;
> > > > > > > +
> > > > > > > +    ASSERT(!local_irq_is_enabled());
> > > > > > > +
> > > > > > > +    while ((i = find_next_bit((const long unsigned int *)
> > > > > > > &this_cpu(lr_mask),
> > > > > > > +                              nr_lrs, i)) < nr_lrs) {
> > > > > > 
> > > > > > Did you look at to ELRSR{0,1} registers which list the usable LRs? 
> > > > > > I think you
> > > > > > can use it with the this_cpu(lr_mask) to avoid browsing every LRs.
> > > > > 
> > > > > Given that we only have 4 LR registers, I think that unconditionally
> > > > > reading 2 ELRSR registers would cost more than simply checking lr_mask
> > > > > on average.
> > > > 
> > > > You also need to actually read the LR and do some bit masking etc don't
> > > > you?
> > > 
> > > No bit masking but I need to read the LRs, that are just 4.
> > 
> > Having read them then what do you do with the result? Surely you need to
> > examine some or all of the bits to determine if it is free or not?
> 
> Right:
> 
> if ( !(lr & (GICH_LR_PENDING|GICH_LR_ACTIVE)) )

This is basically exactly what the ELRSR registers do for you.

Why not just use the h/w feature which gives you exactly what you need
instead of reinventing it yourself because you think it might be faster?
I think there is a certain amount of premature optimisation going on
here.

Ian.


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