linux-kernel - Re: [PATCH] iommu/amd: flush not present cache in iommu_map

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Date:   Wed, 24 Apr 2019 15:47:39 +0100
From:   Tom Murphy <tmurphy@...sta.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Tom Murphy <murphyt7@....ie>
Subject: Re: [PATCH] iommu/amd: flush not present cache in iommu_map_page

>The two conditions can go into one if statement to make this a little
>more clear.
Ah, yeah of course

>And I'd really like to understand the unlikely - amd_iommu_np_cache
>is set based on a hardware capability, so it seems rather odd to mark
>it unlikely.  Dynamic branch prediction really should do the right thing
>here usually.

Here is the commit which added it without any explanation:
https://github.com/torvalds/linux/commit/270cab2426cdc6307725e4f1f46ecf8ab8e69193

should we remove it seen as there's no explanation given ?


On Wed, Apr 24, 2019 at 3:32 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Wed, Apr 24, 2019 at 03:18:59PM +0100, Tom Murphy via iommu wrote:
> > check if there is a not-present cache present and flush it if there is.
> >
> > Signed-off-by: Tom Murphy <tmurphy@...sta.com>
> > ---
> >  drivers/iommu/amd_iommu.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
> > index f7cdd2ab7f11..8ef43224aae0 100644
> > --- a/drivers/iommu/amd_iommu.c
> > +++ b/drivers/iommu/amd_iommu.c
> > @@ -1636,6 +1636,12 @@ static int iommu_map_page(struct protection_domain *dom,
> >               pte[i] = __pte;
> >
> >       update_domain(dom);
> > +     if (!dom->updated) {
> > +             if (unlikely(amd_iommu_np_cache)) {
> > +                     domain_flush_pages(dom, bus_addr, page_size);
> > +                     domain_flush_complete(dom);
> > +             }
> > +     }
>
> The two conditions can go into one if statement to make this a little
> more clear.
>
> And I'd really like to understand the unlikely - amd_iommu_np_cache
> is set based on a hardware capability, so it seems rather odd to mark
> it unlikely.  Dynamic branch prediction really should do the right thing
> here usually.