Re: [PATCH 3/6] iommu/vt-d: Don't force 32bit devices to uses DMA domain

[Lu Baolu]

Hi Christoph,

On 2020/3/7 22:21, Christoph Hellwig wrote:
> On Sat, Mar 07, 2020 at 02:20:11PM +0800, Lu Baolu wrote:
> > Currently, if a 32bit device initially uses an identity domain,
> > Intel IOMMU driver will convert it forcibly to a DMA one if its
> > address capability is not enough for the whole system memory.
> > The motivation was to overcome the overhead caused by possible
> > bounced buffer.
> >
> > Unfortunately, this improvement has led to many problems. For
> > example, some 32bit devices are required to use an identity
> > domain, forcing them to use DMA domain will cause the device
> > not to work anymore. On the other hand, the VMD sub-devices
> > share a domain but each sub-device might have different address
> > capability. Forcing a VMD sub-device to use DMA domain blindly
> > will impact the operation of other sub-devices without any
> > notification. Further more, PCI aliased devices (PCI bridge
> > and all devices beneath it, VMD devices and various devices
> > quirked with pci_add_dma_alias()) must use the same domain.
> > Forcing one device to switch to DMA domain during runtime
> > will cause in-fligh DMAs for other devices to abort or target
> > to other memory which might cause undefind system behavior.
>
> I still don't like the idea to enforce either a strict dynamic
> IOMMU mapping or an identify mapping mode.
>
> Can we add a new AUTO domain which will allow using the identity
> mapping when available?  That somewhat matches the existing x86
> default, and also what powerpc does.

Sai is proposing a series to change the default domain through sysfs
during runtime.

https://lore.kernel.org/linux-iommu/FFF73D592F13FD46B8700F0A279B802F4FBF7E4B@xxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#mb919da5567da7692ee7058a00a137145adf950a1

It has evolved into v2. Not sure whether it's what you want.

> I have a series to lift
> that bypass mode into the core dma-mapping code that I need
> to repost, which I think would be suitable for intel-iommu as well.

Looking forward to your repost.

Best regards,
baolu