Re: [PATCH] bus/pci: forbid VA as IOVA mode if IOMMU address width too small

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon,  8 Jan 2018 14:51:27 +0100
Maxime Coquelin <maxime.coquelin@redhat.com> wrote:

> Intel VT-d supports different address widths for the IOVAs, from
> 39 bits to 56 bits.
> 
> While recent processors support at least 48 bits, VT-d emulation
> currently only supports 39 bits. It makes DMA mapping to fail in this
> case when using VA as IOVA mode, as user-space virtual addresses uses
> up to 47 bits (see kernel's Documentation/x86/x86_64/mm.txt).
> 
> This patch parses VT-d CAP register value available in sysfs, and
> forbid VA as IOVA mode if the GAW is 39 bits or unknown.
> 
> Fixes: f37dfab21c98 ("drivers/net: enable IOVA mode for Intel PMDs")
> 
> Cc: stable@dpdk.org
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
> Hi,
> 
> I'm not super happy with the patch as it does platform specific things in
> generic code, but there are no placeholder for IOMMU/VT-d at the moment.
> 
> As this patch is to be backported to v17.11 LTS, it cannot be a big rework.
> 
> If you have some suggestion to improve it, please let me know.
> 
> The fix is quite urgent, as guest device assignment with vIOMMU is broken in
> mainline & v17.11 LTS.
> 
> Advantage of this fix over forbidding VA as IOVA when running in emulation is
> that VT-d emulation will soon support 48 bits, so this is future proof. Also,
> VT-d spec supports 39 bits, so we could have physical CPUs supporting it, even
> if I don't know any.
> 
> Thanks,
> Maxime

You are assumming that if IOMMU is present that it is being used (ie VFIO).
What about the case of direct access to PF device via IGB_UIO?

> +static inline bool
> +pci_one_device_iommu_support_va(struct rte_pci_device *dev)
> +{

This is not in fast path, there is no reason it should be inline