Re: [PATCH v6 25/34] swiotlb: Add warnings for use of bounce buffers with SME

[Tom Lendacky]

On 6/14/2017 11:50 AM, Borislav Petkov wrote:
> On Wed, Jun 07, 2017 at 02:17:32PM -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.
> >
> > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > ---
> >   arch/x86/include/asm/mem_encrypt.h |    8 ++++++++
> >   include/asm-generic/mem_encrypt.h  |    5 +++++
> >   include/linux/dma-mapping.h        |    9 +++++++++
> >   lib/swiotlb.c                      |    3 +++
> >   4 files changed, 25 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> > index f1215a4..c7a2525 100644
> > --- a/arch/x86/include/asm/mem_encrypt.h
> > +++ b/arch/x86/include/asm/mem_encrypt.h
> > @@ -69,6 +69,14 @@ static inline bool sme_active(void)
> >   	return !!sme_me_mask;
> >   }
> >   
> > +static inline u64 sme_dma_mask(void)
> > +{
> > +	if (!sme_me_mask)
> > +		return 0ULL;
> > +
> > +	return ((u64)sme_me_mask << 1) - 1;
> > +}
> > +
> >   /*
> >    * The __sme_pa() and __sme_pa_nodebug() macros are meant for use when
> >    * writing to or comparing values from the cr3 register.  Having the
> > diff --git a/include/asm-generic/mem_encrypt.h b/include/asm-generic/mem_encrypt.h
> > index b55c3f9..fb02ff0 100644
> > --- a/include/asm-generic/mem_encrypt.h
> > +++ b/include/asm-generic/mem_encrypt.h
> > @@ -22,6 +22,11 @@ static inline bool sme_active(void)
> >   	return false;
> >   }
> >   
> > +static inline u64 sme_dma_mask(void)
> > +{
> > +	return 0ULL;
> > +}
> > +
> >   /*
> >    * The __sme_set() and __sme_clr() macros are useful for adding or removing
> >    * the encryption mask from a value (e.g. when dealing with pagetable
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index 4f3eece..e2c5fda 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 +578,10 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
> >   
> >   	if (!dev->dma_mask || !dma_supported(dev, mask))
> >   		return -EIO;
> > +
> > +	if (sme_active() && (mask < sme_dma_mask()))
> > +		dev_warn(dev, "SME is active, device will require DMA bounce buffers\n");
>
> Something looks strange here:
>
> you're checking sme_active() before calling sme_dma_mask() and yet in
> it, you're checking !sme_me_mask again. What gives?

I guess I don't need the sme_active() check since the second part of the
if statement can only ever be true if SME is active (since mask is
unsigned).

Thanks,
Tom

> Why not move the sme_active() check into sme_dma_mask() and thus
> simplify callers?