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

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Message-ID: <cfe46eda-66b5-b40d-6721-84e6e0e1f5de@amd.com>
Date:   Wed, 4 Sep 2019 07:33:27 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>, Dave Hansen <dave.hansen@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
CC:     dri-devel <dri-devel@...ts.freedesktop.org>,
        "pv-drivers@...are.com" <pv-drivers@...are.com>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Lendacky, Thomas" <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>
Subject: Re: [PATCH v2 3/4] drm/ttm, drm/vmwgfx: Correctly support support AMD
 memory encryption

Am 03.09.19 um 23:05 schrieb Thomas Hellström (VMware):
> On 9/3/19 10:51 PM, Dave Hansen wrote:
>> 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?
>
> With SEV I think that we could possibly establish the encryption flags 
> at vma creation time. But thinking of it, it would actually break with 
> SME where buffer content can be moved between encrypted system memory 
> and unencrypted graphics card PCI memory behind user-space's back. 
> That would imply killing all user-space encrypted PTEs and at fault 
> time set up new ones pointing to unencrypted PCI memory..

Well my problem is where do you see encrypted system memory here?

At least for AMD GPUs all memory accessed must be unencrypted and that 
counts for both system as well as PCI memory.

So I don't get why we can't assume always unencrypted and keep it like that.

Regards,
Christian.