Re: [Xen-devel] [PATCH v4 3/4] xen: implement guest_physmap_(un)pin_range

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 19 Aug 2013, Ian Campbell wrote:
> > +{
> > +    lpae_t pte = *ptep;
> > +    if ( pte.p2m.avail & P2M_DMA_PIN )
> > +        return -EINVAL;
> > +    pte.p2m.avail |= P2M_DMA_PIN;
> 
> I think test_and_set_bit would be better here and you should operate on
> ptep directly IMHO rather than changing a copy and writing that. Might
> require a pte_modified macro with the flushing stuff from write_pte.
> 

Considering that ptep->p2m.avail is a bit-field and that
test_and_set_bit operates on a unsigned long pointer (one cannot get a
pointer to a bit-field), and considering that avail is at bit 55 > 32
(we would need to take care of incrementing the pointer too), I think
that it's not worth making this change because it would make the code
far less readable.

In any case these two functions are called with the p2m lock held, so
the current implementation should be safe. 

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