On 1/6/22 11:46 PM, Jason Gunthorpe wrote:
On Thu, Jan 06, 2022 at 11:54:06AM +0800, Lu Baolu wrote:
On 1/5/22 3:23 AM, Jason Gunthorpe wrote:
The device driver oriented interfaces are,
int iommu_device_use_dma_api(struct device *dev);
void iommu_device_unuse_dma_api(struct device *dev);
Nit, do we care whether it uses the actual DMA API? Or is it just
that iommu_device_use_dma_api() tells us the driver may program the
device to do DMA?
As the main purpose, yes this is all about the DMA API because it
asserts the group domain is the DMA API's domain.
There is a secondary purpose that has to do with the user/kernel
attack you mentioned above. Maintaining the DMA API domain also
prevents VFIO from allowing userspace to operate any device in the
group which blocks P2P attacks to MMIO of other devices.
This is why, even if the driver doesn't use DMA, it should still do a
iommu_device_use_dma_api(), except in the special cases where we don't
care about P2P attacks (eg pci-stub, bridges, etc).
By the way, use_dma_api seems hard to read. How about
iommu_device_use_default_dma()?
You could just say "use default domain"
IMHO the way the iommu subsystem has its own wonky language is a
little troublesome. In the rest of the kernel we call this the DMA
API, while the iommu subsystem calls the domain that the DMA API uses
the 'default domain' not the 'DMA API' domain.
Still, it is probably better to align to the iommu language - just be
sure to put in the function comment that this API 'allows the driver
to use the DMA API eg dma_map_sg()'
iommu_device_use_default_domain() reads better. And add some comments
to link "default domain" with "DMA API". Thanks!
Jason
Best regards,
baolu
