Re: [PATCH V2 1/3] scsi: mptxsas: try 64 bit DMA when 32 bit DMA fails

From: Sinan Kaya
Date: Tue Nov 10 2015 - 15:58:29 EST

On 11/10/2015 2:56 PM, Arnd Bergmann wrote:
The ACPI IORT table declares whether you enable IOMMU for a particular
>device or not. The placement of IOMMU HW is system specific. The IORT
>table gives the IOMMU HW topology to the operating system.
This sounds odd. Clearly you need to specify the IOMMU settings for each
possible PCI device independent of whether the OS actually uses the IOMMU
or not.

There are provisions to have DMA mask in the PCIe host bridge not at the PCIe device level inside IORT table. This setting is specific for each PCIe bus. It is not per PCIe device.

It is assumed that the endpoint device driver knows the hardware for PCIe devices. The driver can also query the supported DMA bits by this platform via DMA APIs and will request the correct DMA mask from the DMA subsystem (surprise!).

In a lot of cases, we want to turn it off to get better performance
when the driver has set a DMA mask that covers all of RAM, but you
also want to enable the IOMMU for debugging purposes or for device
assignment if you run virtual machines. The bootloader doesn't know how
the device is going to be used, so it cannot define the policy here.

I think we'll end up adding a virtualization option to the UEFI BIOS similar to how Intel platforms work. Based on this switch, we'll end up patching the ACPI table.

If I remove the IORT entry, then the device is in coherent mode with device accessing the full RAM range.

If I have the IORT table, the device is in IOMMU translation mode.

Details are in the IORT spec.

Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
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