linux-kernel - Re: [PATCH 5/5] virtio: Add bounce DMA ops

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Date:   Wed, 29 Apr 2020 13:42:13 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     baolu.lu@...ux.intel.com,
        Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        tsoni@...eaurora.org, virtio-dev@...ts.oasis-open.org,
        konrad.wilk@...cle.com, jan.kiszka@...mens.com,
        jasowang@...hat.com, christoffer.dall@....com,
        virtualization@...ts.linux-foundation.org, alex.bennee@...aro.org,
        iommu@...ts.linux-foundation.org, stefano.stabellini@...inx.com,
        will@...nel.org, linux-kernel@...r.kernel.org,
        pratikp@...eaurora.org
Subject: Re: [PATCH 5/5] virtio: Add bounce DMA ops

On 2020/4/29 12:57, Michael S. Tsirkin wrote:
> On Wed, Apr 29, 2020 at 10:22:32AM +0800, Lu Baolu wrote:
>> On 2020/4/29 4:41, Michael S. Tsirkin wrote:
>>> On Tue, Apr 28, 2020 at 11:19:52PM +0530, Srivatsa Vaddagiri wrote:
>>>> * Michael S. Tsirkin<mst@...hat.com>  [2020-04-28 12:17:57]:
>>>>
>>>>> Okay, but how is all this virtio specific?  For example, why not allow
>>>>> separate swiotlbs for any type of device?
>>>>> For example, this might make sense if a given device is from a
>>>>> different, less trusted vendor.
>>>> Is swiotlb commonly used for multiple devices that may be on different trust
>>>> boundaries (and not behind a hardware iommu)?
>>> Even a hardware iommu does not imply a 100% security from malicious
>>> hardware. First lots of people use iommu=pt for performance reasons.
>>> Second even without pt, unmaps are often batched, and sub-page buffers
>>> might be used for DMA, so we are not 100% protected at all times.
>>>
>>
>> For untrusted devices, IOMMU is forced on even iommu=pt is used;
> 
> I think you are talking about untrusted *drivers* like with VFIO.

No. I am talking about untrusted devices like thunderbolt peripherals.
We always trust drivers hosted in kernel and the DMA APIs are designed
for them, right?

Please refer to this series.

https://lkml.org/lkml/2019/9/6/39

Best regards,
baolu

> 
> On the other hand, I am talking about things like thunderbolt
> peripherals being less trusted than on-board ones.



> 
> Or possibly even using swiotlb for specific use-cases where
> speed is less of an issue.
> 
> E.g. my wifi is pretty slow anyway, and that card is exposed to
> malicious actors all the time, put just that behind swiotlb
> for security, and leave my graphics card with pt since
> I'm trusting it with secrets anyway.
> 
> 
>> and
>> iotlb flush is in strict mode (no batched flushes); ATS is also not
>> allowed. Swiotlb is used to protect sub-page buffers since IOMMU can
>> only apply page granularity protection. Swiotlb is now used for devices
>> from different trust zone.
>>
>> Best regards,
>> baolu
>