linux-kernel - Re: AMD SME encrpytion and PCI BAR pages to user space

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Message-ID: <20201027115810.GH1523783@nvidia.com>
Date:   Tue, 27 Oct 2020 08:58:10 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Tom Lendacky <thomas.lendacky@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <kvm@...r.kernel.org>,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Alexander Potapenko" <glider@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Dmitry Vyukov" <dvyukov@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: AMD SME encrpytion and PCI BAR pages to user space

On Tue, Oct 27, 2020 at 08:43:57AM +0000, Christoph Hellwig wrote:
> On Wed, Oct 21, 2020 at 01:03:22PM -0300, Jason Gunthorpe wrote:
> > Oh, interesting.. Yes the issue is no userspace DMA stuff uses the DMA
> > API correctly (because it is in userspace)
> > 
> > So SWIOTLB tricks don't work, I wish the dma_map could fail for these
> > situations
> 
> Userspace DMA by definition also does not use dma_map..

? Sure it does, ib_dma_map_sg_attrs() is what RDMA uses

What all the userspace users skip is the dma_sync*() - that would
require a kernel call which defeats the point.

So, my desire is some flag to dma_map_sg() that says
  'user space mapping no dma_sync_*'

ie dma_sync_* is a NOP

Then things like SWIOTLB on the SEV system can fail with an error code
instead of malfunctioning

If FOLL_LONGERM is some estimate of this pattern then we have these users:
 - drivers/infiniband
 - v4l
 - vdpa
 - xdp
 - rds
 - habana labs

Jason