Re: [PATCH v7 25/36] swiotlb: Add warnings for use of bounce buffers with SME

[Tom Lendacky]

On 6/21/2017 5:50 AM, Borislav Petkov wrote:
> On Fri, Jun 16, 2017 at 01:54:36PM -0500, Tom Lendacky wrote:
> > Add warnings to let the user know when bounce buffers are being used for
> > DMA when SME is active.  Since the bounce buffers are not in encrypted
> > memory, these notifications are to allow the user to determine some
> > appropriate action - if necessary.  Actions can range from utilizing an
> > IOMMU, replacing the device with another device that can support 64-bit
> > DMA, ignoring the message if the device isn't used much, etc.
> >
> > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > ---
> >   include/linux/dma-mapping.h |   11 +++++++++++
> >   include/linux/mem_encrypt.h |    8 ++++++++
> >   lib/swiotlb.c               |    3 +++
> >   3 files changed, 22 insertions(+)
> >
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index 4f3eece..ee2307e 100644
> > --- a/include/linux/dma-mapping.h
> > +++ b/include/linux/dma-mapping.h
> > @@ -10,6 +10,7 @@
> >   #include <linux/scatterlist.h>
> >   #include <linux/kmemcheck.h>
> >   #include <linux/bug.h>
> > +#include <linux/mem_encrypt.h>
> >   
> >   /**
> >    * List of possible attributes associated with a DMA mapping. The semantics
> > @@ -577,6 +578,11 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
> >   
> >   	if (!dev->dma_mask || !dma_supported(dev, mask))
> >   		return -EIO;
> > +
> > +	/* Since mask is unsigned, this can only be true if SME is active */
> > +	if (mask < sme_dma_mask())
> > +		dev_warn(dev, "SME is active, device will require DMA bounce buffers\n");
> > +
> >   	*dev->dma_mask = mask;
> >   	return 0;
> >   }
> > @@ -596,6 +602,11 @@ static inline int dma_set_coherent_mask(struct device *dev, u64 mask)
> >   {
> >   	if (!dma_supported(dev, mask))
> >   		return -EIO;
> > +
> > +	/* Since mask is unsigned, this can only be true if SME is active */
> > +	if (mask < sme_dma_mask())
> > +		dev_warn(dev, "SME is active, device will require DMA bounce buffers\n");
>
> Looks to me like those two checks above need to be a:
>
> void sme_check_mask(struct device *dev, u64 mask)
> {
>          if (!sme_me_mask)
>                  return;
>
>          /* Since mask is unsigned, this can only be true if SME is active */
>          if (mask < (((u64)sme_me_mask << 1) - 1))
>                  dev_warn(dev, "SME is active, device will require DMA bounce buffers\n");
> }
>
> which gets called and sme_dma_mask() is not really needed.

Makes a lot of sense, I'll update the patch.

Thanks,
Tom

>