Subject: Re: [RFC PATCH v3 13/20] x86: DMA support for memory encryption
To: "Michael S. Tsirkin" <mst@redhat.com>
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From: Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <4d97f998-5835-f4e0-9840-7f7979251275@amd.com>
Date: Tue, 15 Nov 2016 12:29:35 -0600
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On 11/15/2016 9:16 AM, Michael S. Tsirkin wrote:
> On Wed, Nov 09, 2016 at 06:37:23PM -0600, Tom Lendacky wrote:
>> Since DMA addresses will effectively look like 48-bit addresses when the
>> memory encryption mask is set, SWIOTLB is needed if the DMA mask of the
>> device performing the DMA does not support 48-bits. SWIOTLB will be
>> initialized to create un-encrypted bounce buffers for use by these devices.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/include/asm/dma-mapping.h |    5 ++-
>>  arch/x86/include/asm/mem_encrypt.h |    5 +++
>>  arch/x86/kernel/pci-dma.c          |   11 ++++---
>>  arch/x86/kernel/pci-nommu.c        |    2 +
>>  arch/x86/kernel/pci-swiotlb.c      |    8 ++++-
>>  arch/x86/mm/mem_encrypt.c          |   17 +++++++++++
>>  include/linux/swiotlb.h            |    1 +
>>  init/main.c                        |   13 ++++++++
>>  lib/swiotlb.c                      |   58 +++++++++++++++++++++++++++++++-----
>>  9 files changed, 103 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/dma-mapping.h b/arch/x86/include/asm/dma-mapping.h
>> index 4446162..c9cdcae 100644
>> --- a/arch/x86/include/asm/dma-mapping.h
>> +++ b/arch/x86/include/asm/dma-mapping.h

..SNIP...

>>  
>> +/*
>> + * If memory encryption is active, the DMA address for an encrypted page may
>> + * be beyond the range of the device. If bounce buffers are required be sure
>> + * that they are not on an encrypted page. This should be called before the
>> + * iotlb area is used.
> 
> Makes sense, but I think at least a dmesg warning here
> might be a good idea.

Good idea.  Should it be a warning when it is first being set up or
a warning the first time the bounce buffers need to be used.  Or maybe
both?

> 
> A boot flag that says "don't enable devices that don't support
> encryption" might be a good idea, too, since most people
> don't read dmesg output and won't notice the message.

I'll look into this. It might be something that can be checked as
part of the device setting its DMA mask or the first time a DMA
API is used if the device doesn't explicitly set its mask.

Thanks,
Tom

> 
>