[RFC PATCH v3 0/6] Restricted DMA

[Florian Fainelli]

On 1/11/21 11:48 PM, Claire Chang wrote:
> On Fri, Jan 8, 2021 at 1:59 AM Florian Fainelli <f.fainelli at gmail.com> wrote:
>>
>> On 1/7/21 9:42 AM, Claire Chang wrote:
>>
>>>> Can you explain how ATF gets involved and to what extent it does help,
>>>> besides enforcing a secure region from the ARM CPU's perpsective? Does
>>>> the PCIe root complex not have an IOMMU but can somehow be denied access
>>>> to a region that is marked NS=0 in the ARM CPU's MMU? If so, that is
>>>> still some sort of basic protection that the HW enforces, right?
>>>
>>> We need the ATF support for memory MPU (memory protection unit).
>>> Restricted DMA (with reserved-memory in dts) makes sure the predefined memory
>>> region is for PCIe DMA only, but we still need MPU to locks down PCIe access to
>>> that specific regions.
>>
>> OK so you do have a protection unit of some sort to enforce which region
>> in DRAM the PCIE bridge is allowed to access, that makes sense,
>> otherwise the restricted DMA region would only be a hint but nothing you
>> can really enforce. This is almost entirely analogous to our systems then.
> 
> Here is the example of setting the MPU:
> https://github.com/ARM-software/arm-trusted-firmware/blob/master/plat/mediatek/mt8183/drivers/emi_mpu/emi_mpu.c#L132
> 
>>
>> There may be some value in standardizing on an ARM SMCCC call then since
>> you already support two different SoC vendors.
>>
>>>
>>>>
>>>> On Broadcom STB SoCs we have had something similar for a while however
>>>> and while we don't have an IOMMU for the PCIe bridge, we do have a a
>>>> basic protection mechanism whereby we can configure a region in DRAM to
>>>> be PCIe read/write and CPU read/write which then gets used as the PCIe
>>>> inbound region for the PCIe EP. By default the PCIe bridge is not
>>>> allowed access to DRAM so we must call into a security agent to allow
>>>> the PCIe bridge to access the designated DRAM region.
>>>>
>>>> We have done this using a private CMA area region assigned via Device
>>>> Tree, assigned with a and requiring the PCIe EP driver to use
>>>> dma_alloc_from_contiguous() in order to allocate from this device
>>>> private CMA area. The only drawback with that approach is that it
>>>> requires knowing how much memory you need up front for buffers and DMA
>>>> descriptors that the PCIe EP will need to process. The problem is that
>>>> it requires driver modifications and that does not scale over the number
>>>> of PCIe EP drivers, some we absolutely do not control, but there is no
>>>> need to bounce buffer. Your approach scales better across PCIe EP
>>>> drivers however it does require bounce buffering which could be a
>>>> performance hit.
>>>
>>> Only the streaming DMA (map/unmap) needs bounce buffering.
>>
>> True, and typically only on transmit since you don't really control
>> where the sk_buff are allocated from, right? On RX since you need to
>> hand buffer addresses to the WLAN chip prior to DMA, you can allocate
>> them from a pool that already falls within the restricted DMA region, right?
>>
> 
> Right, but applying bounce buffering to RX will make it more secure.
> The device won't be able to modify the content after unmap. Just like what
> iommu_unmap does.

Sure, however the goals of using bounce buffering equally applies to RX
and TX in that this is the only layer sitting between a stack (block,
networking, USB, etc.) and the underlying device driver that scales well
in order to massage a dma_addr_t to be within a particular physical range.

There is however room for improvement if the drivers are willing to
change their buffer allocation strategy. When you receive Wi-Fi frames
you need to allocate buffers for the Wi-Fi device to DMA into, and that
happens ahead of the DMA transfers by the Wi-Fi device. At buffer
allocation time you could very well allocate these frames from the
restricted DMA region without having to bounce buffer them since the
host CPU is in control over where and when to DMA into.

The issue is that each network driver may implement its own buffer
allocation strategy, some may simply call netdev_alloc_skb() which gives
zero control over where the buffer comes from unless you play tricks
with NUMA node allocations and somehow declare that your restricted DMA
region is a different NUMA node. If the driver allocates pages and then
attaches a SKB to that page using build_skb(), then you have much more
control over where that page comes from, and this is where using a
device private CMA are helps, because you can just do
dma_alloc_from_contiguous() and that will ensure that the pages are
coming from your specific CMA area.

Few questions on the implementation:

- is there any warning or error being printed if the restricted DMA
region is outside of a device's DMA addressable range?

- are there are any helpful statistics that could be shown to indicate
that the restricted DMA region was sized too small, e.g.: that
allocation of a DMA buffer failed because we ran out of space in the
swiotlb pool?

> 
>>> I also added alloc/free support in this series
>>> (https://lore.kernel.org/patchwork/patch/1360995/), so dma_direct_alloc() will
>>> try to allocate memory from the predefined memory region.
>>>
>>> As for the performance hit, it should be similar to the default swiotlb.
>>> Here are my experiment results. Both SoCs lack IOMMU for PCIe.
>>>
>>> PCIe wifi vht80 throughput -
>>>
>>>   MTK SoC                  tcp_tx     tcp_rx    udp_tx   udp_rx
>>>   w/o Restricted DMA  244.1     134.66   312.56   350.79
>>>   w/ Restricted DMA    246.95   136.59   363.21   351.99
>>>
>>>   Rockchip SoC           tcp_tx     tcp_rx    udp_tx   udp_rx
>>>   w/o Restricted DMA  237.87   133.86   288.28   361.88
>>>   w/ Restricted DMA    256.01   130.95   292.28   353.19
>>
>> How come you get better throughput with restricted DMA? Is it because
>> doing DMA to/from a contiguous region allows for better grouping of
>> transactions from the DRAM controller's perspective somehow?
> 
> I'm not sure, but actually, enabling the default swiotlb for wifi also helps the
> throughput a little bit for me.

OK, it would be interesting if you could get to the bottom of why
performance does increase with swiotlb.
-- 
Florian