linux-kernel - Re: [RFC PATCH v3 13/20] x86: DMA support for memory encryption

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Date:   Tue, 15 Nov 2016 21:16:33 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     linux-arch@...r.kernel.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org, x86@...nel.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>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFC PATCH v3 13/20] x86: DMA support for memory encryption

On Tue, Nov 15, 2016 at 12:29:35PM -0600, Tom Lendacky wrote:
> 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@....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
> 
> > 

I think setup time is nicer if it's possible.