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

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Message-ID: <7fa3b178-b9b4-2df9-1eee-54e24d48342e@intel.com>
Date:   Tue, 3 Sep 2019 13:51:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>, Daniel Vetter <daniel@...ll.ch>
Cc:     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>,
        Andy Lutomirski <luto@...nel.org>,
        "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 9/3/19 1:36 PM, Thomas Hellström (VMware) wrote:
> So the question here should really be, can we determine already at mmap
> time whether backing memory will be unencrypted and adjust the *real*
> vma->vm_page_prot under the mmap_sem?
> 
> Possibly, but that requires populating the buffer with memory at mmap
> time rather than at first fault time.

I'm not connecting the dots.

vma->vm_page_prot is used to create a VMA's PTEs regardless of if they
are created at mmap() or fault time.  If we establish a good
vma->vm_page_prot, can't we just use it forever for demand faults?

Or, are you concerned that if an attempt is made to demand-fault page
that's incompatible with vma->vm_page_prot that we have to SEGV?

> And it still requires knowledge whether the device DMA is always
> unencrypted (or if SEV is active).

I may be getting mixed up on MKTME (the Intel memory encryption) and
SEV.  Is SEV supported on all memory types?  Page cache, hugetlbfs,
anonymous?  Or just anonymous?