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

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Message-ID: <7eec2c11-d0d4-4c81-6ed2-d0932bf5af33@amd.com>
Date:   Wed, 4 Sep 2019 11:10:34 +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 04.09.19 um 10:19 schrieb Thomas Hellström (VMware):
> Hi, Christian,
>
> On 9/4/19 9:33 AM, Koenig, Christian wrote:
>> 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.
>
> We're talking SME now right?
>
> The current SME setup is that if a device's DMA mask says it's capable 
> of addressing the encryption bit, coherent memory will be encrypted. 
> The memory controllers will decrypt for the device on the fly. 
> Otherwise coherent memory will be decrypted.
>
>>
>> So I don't get why we can't assume always unencrypted and keep it 
>> like that.
>
> I see two reasons. First, it would break with a real device that 
> signals it's capable of addressing the encryption bit.

Why? Because we don't use dma_mmap_coherent()?

I've already talked with Christoph that we probably want to switch TTM 
over to using that instead to also get rid of the ttm_io_prot() hack.

Regards,
Christian.

>
> Second I can imagine unaccelerated setups (something like vkms using 
> prime feeding a VNC connection) where we actually want the TTM buffers 
> encrypted to protect data.
>
> But at least the latter reason is way far out in the future.
>
> So for me I'm ok with that if that works for you?
>
> /Thomas
>
>
>>
>> Regards,
>> Christian.
>
>