linux-kernel - Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD memory encryption

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Date:   Wed, 4 Sep 2019 11:16:37 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        pv-drivers@...are.com,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support
 AMD memory encryption

On Wed, Sep 04, 2019 at 08:49:03AM +0200, Thomas Hellström (VMware) wrote:
> For device DMA address purposes, the encryption status is encoded in the dma
> address by the dma layer in phys_to_dma().
> 
> 
> >   There doesn’t seem to be any real funny business in dma_mmap_attrs() or dma_common_mmap().
> 
> No, from what I can tell the call in these functions to dma_pgprot()
> generates an incorrect page protection since it doesn't take unencrypted
> coherent memory into account. I don't think anybody has used these functions
> yet with SEV.

Yes, I think dma_pgprot is not correct for SEV.  Right now that function
isn't used much on x86, it had more grave bugs up to a few -rcs ago..

> > Would it make sense to add a vmf_insert_dma_page() to directly do exactly what you’re trying to do?
> 
> Yes, but as a longer term solution I would prefer a general dma_pgprot()
> exported, so that we could, in a dma-compliant way, use coherent pages with
> other apis, like kmap_atomic_prot() and vmap(). That is, basically split
> coherent page allocation in two steps: Allocation and mapping.

The thing is that dma_pgprot is of no help for you at all, as the DMA
API hides the page from you entirely.  In fact we do have backends that
do not even have a page backing.  But I think we can have a
vmf_insert_page equivalent that does the right thing behind your back
for the varius different implementation (contiguous page(s) in the kernel
lineary, contiguous page(s) with a vmap/ioremap remapping in various
flavours, non-contigous pages(s) with a vmap remapping, and deeply
magic firmware populated pools (well, except maybe for the last, but
at least we can fail gracefully there)).