[RFC PATCH v3 0/6] Restricted DMA

[Claire Chang]

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.

> > 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.

>
> >
> > The CPU usage doesn't increase too much either.
> > Although I didn't measure the CPU usage very precisely, it's ~3% with a single
> > big core (Cortex-A72) and ~5% with a single small core (Cortex-A53).
> >
> > Thanks!
> >
> >>
> >> Thanks!
> >> --
> >> Florian
>
>
> --
> Florian