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

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Date:   Wed, 14 Jun 2017 14:49:02 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-arch@...r.kernel.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org, Rik van Riel <riel@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dave Young <dyoung@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v6 25/34] swiotlb: Add warnings for use of bounce buffers
 with SME

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@....com>
>> ---
>>   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?
>