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

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Message-ID: <80a3a471-0a2c-3ab3-529c-1b8b624679f8@linux.intel.com>
Date:   Wed, 29 Apr 2020 15:01:14 +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 14:50, Michael S. Tsirkin wrote:
> On Wed, Apr 29, 2020 at 01:42:13PM +0800, Lu Baolu wrote:
>> 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
> Oh, thanks for that! I didn't realize Linux is doing this.
> 
> So it seems that with modern Linux, all one needs
> to do on x86 is mark the device as untrusted.
> It's already possible to do this with ACPI and with OF - would that be
> sufficient for achieving what this patchset is trying to do?

Yes.

> 
> Adding more ways to mark a device as untrusted, and adding
> support for more platforms to use bounce buffers
> sounds like a reasonable thing to do.
> 

Agreed.

Best regards,
baolu