linux-kernel - Re: Regression due to d98849aff879 (dma-direct: handle DMA_ATTR_NO_KERNEL

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Date:   Tue, 6 Aug 2019 13:33:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: Regression due to d98849aff879 (dma-direct: handle
 DMA_ATTR_NO_KERNEL_MAPPING in common code)

On Tue, Aug 06, 2019 at 11:13:29AM +0200, Lucas Stach wrote:
> Hi Christoph,
> 
> I just found a regression where my NVMe device is no longer able to set
> up its HMB.
> 
> After subject commit dma_direct_alloc_pages() is no longer initializing
> dma_handle properly when DMA_ATTR_NO_KERNEL_MAPPING is set, as the
> function is now returning too early.
> 
> Now this could easily be fixed by adding the phy_to_dma translation to
> the NO_KERNEL_MAPPING code path, but I'm not sure how this stuff
> interacts with the memory encryption stuff set up later in the
> function, so I guess this should be looked at by someone with more
> experience with this code than me.

There is not much we can do about the memory encryption case here,
as that requires a kernel address to mark the memory as unencrypted.

So the obvious trivial fix is probably the right one:


diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 59bdceea3737..c49120193309 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -135,6 +135,7 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size,
 		if (!PageHighMem(page))
 			arch_dma_prep_coherent(page, size);
 		/* return the page pointer as the opaque cookie */
+		*dma_handle = phys_to_dma(dev, page_to_phys(page));
 		return page;
 	}