Re: [PATCH v2] iommu/arm-smmu: Return IOVA in iova_to_phys when SMMU is bypassed

[Will Deacon]

On Wed, Apr 26, 2017 at 04:13:29PM +0530, Sunil Kovvuri wrote:
> On Wed, Apr 26, 2017 at 3:31 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> > Hi Sunil,
> >
> > On Tue, Apr 25, 2017 at 03:27:52PM +0530, sunil.kovvuri@xxxxxxxxx wrote:
> >> From: Sunil Goutham <sgoutham@xxxxxxxxxx>
> >>
> >> For software initiated address translation, when domain type is
> >> IOMMU_DOMAIN_IDENTITY i.e SMMU is bypassed, mimic HW behavior
> >> i.e return the same IOVA as translated address.
> >>
> >> This patch is an extension to Will Deacon's patchset
> >> "Implement SMMU passthrough using the default domain".
> >>
> >> Signed-off-by: Sunil Goutham <sgoutham@xxxxxxxxxx>
> >> ---
> >>
> >> V2
> >> - As per Will's suggestion applied fix to SMMUv3 driver as well.
> >
> > This follows what the AMD driver does, so:
> >
> > Acked-by: Will Deacon <will.deacon@xxxxxxx>
> 
> Thanks,
> 
> >
> > but I still think that having drivers/net/ethernet/cavium/thunder/nicvf_queues.c
> > poke around with the physical address to get at the struct pages underlying
> > a DMA buffer is really dodgy.
> 
> Driver is not dealing with page structures to be precise, just like
> for any other NIC device, driver needs to know the virtual address
> of the packet to where it's DMA'ed, so that SKB if framed and
> handed over to network stack. Due to reasons mentioned below,
> in this driver it's not possible to maintain a list of DMA addresses to
> Virtual address mappings. Hence using IOMMU API, DMA address
> is translated to physical address and finally to virtual address. I don't
> see anything dodgy here.

It's dodgy because you're the only NIC driver using iommu_iova_to_phys
directly and, afaict, the driver could just stash either the struct page
or the virtual address at the point of allocation.

> > Is there no way this can be avoided, perhaps by tracking the pages some other way
> 
> I have explained that in the commit message
> --
>     Also VNIC doesn't have a seperate receive buffer ring per receive
>     queue, so there is no 1:1 descriptor index matching between CQE_RX
>     and the index in buffer ring from where a buffer has been used for
>     DMA'ing. Unlike other NICs, here it's not possible to maintain dma
>     address to virt address mappings within the driver. This leaves us
>     no other choice but to use IOMMU's IOVA address conversion API to
>     get buffer's virtual address which can be given to network stack
>     for processing.
> --
> 
> >(although I don't understand why you're having to mess with the page reference
> >counts to start with)?
> Not sure why you say it's a mess, adjusting page reference counts is quite
> common if you check other NIC drivers. On ARM64 especially when using
> 64KB pages, if we have only one packet buffer for each page then we
> will have to set aside a whole lot of memory which sometimes is not possible
> on embedded platforms. Hence multiple pkt buffers per page, and page reference
> is set accordingly.

I wasn't saying that was a mess, I was just saying that I didn't understand
why you mess (verb) with the page reference counts (my ignorance of the
network layer). The code that I think is a mess is:

		phys_addr = nicvf_iova_to_phys(nic, buf_addr);
		[...]
		put_page(virt_to_page(phys_to_virt(phys_addr)));

because:

  (a) You have the information you need at allocation time, but you've
      failed to record that and are trying to use the IOMMU API to
      reconstruct the CPU virtual address

  (b) When there isn't an IOMMU present, you assume that bus addresses ==
      physical addresses

  (c) You assume that the DMA buffer is mapped in the linear mapping

that's probably all true for ThunderX/arm64, but it's generally not portable
or reliable code. If you could get a handle to the struct page that you
allocated in the first place, then you could use page_address to get its
virtual address instead of having to go via the physical address.

Will