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

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Message-ID: <20190424143246.GA24079@infradead.org>
Date:   Wed, 24 Apr 2019 07:32:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tom Murphy <tmurphy@...sta.com>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        murphyt7@....ie
Subject: Re: [PATCH] iommu/amd: flush not present cache in iommu_map_page

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.