Difference between IOVA and bus address when SMMU is enabled

Difference between IOVA and bus address when SMMU is enabled

[valmiki]

Hi All,

What is the difference between IOVA address and bus address
when SMMU is enabled ?

Is IOVA address term used only when hypervisor is present ?

Regards,
Valmiki

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

Re: Difference between IOVA and bus address when SMMU is enabled

[Jean-Philippe Brucker]

Hi Valmiki,

On 12/05/18 13:55, valmiki wrote:
> Hi All,
> 
> What is the difference between IOVA address and bus address
> when SMMU is enabled ?

They are the same. You'll use one term or the other depending on what
system component you're talking about. "IOVA" only means something when
talking about IOMMUs, where it represents the input address. If you're
discussing bus transactions without caring whether an SMMU is enabled or
not, then "bus address" makes more sense.

We distinguish "IOVA" from "VA", which represents the input address of
the CPU's MMU (e.g. any userspace pointer). The distinction is useful
because the SMMU page tables are usually separate from the CPU page
tables. In this case if you want to share a buffer between application
and device, you'll have to allocate and map both a VA on the CPU side,
and an IOVA on the device side. When sharing MMU page tables with the
SMMU (see the SVA work), then we tend to talk about VA instead of IOVA,
because they are identical.

> Is IOVA address term used only when hypervisor is present ?

No, the term is used in bare-metal setups as well.

Thanks,
Jean

Re: Difference between IOVA and bus address when SMMU is enabled

[Russell King - ARM Linux]

On Sat, May 12, 2018 at 06:25:13PM +0530, valmiki wrote:
> Hi All,
> 
> What is the difference between IOVA address and bus address
> when SMMU is enabled ?
> 
> Is IOVA address term used only when hypervisor is present ?

IOVA = IO virtual address.  IOVA is the term normally used to describe
the address used on the _device_ side of an IOMMU.

For any general setup:

RAM ----- MMU ----- DEVICE
      ^         ^
  physical   virtual
  address    address

where "device" can be an IO device or a CPU, the terms still apply.

If you have something like this:

RAM ----- PCI bridge ----- MMU ----- DEVICE
      ^                ^         ^
   physical           bus     virtual
   address          address   address

You could also have (eg, in the case of a system MMU):

RAM ----- MMU ----- PCI bridge ----- DEVICE
      ^         ^                ^
   physical  virtual            bus
   address   address          address
           (this can also be
            considered a bus
            address!)

In both of the above two cases, the PCI bridge may perform some address
translation, meaning that the bus address is different from the address
seen on the other side of the bridge.

So, the terms used depend exactly on the overall bus topology.

In the case of a system MMU, where the system MMU sits between peripheral
devices and RAM, then the bus addresses are the same as the
_IOVA of the system MMU_.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to speedtest.net: 8.21Mbps down 510kbps up

Re: Difference between IOVA and bus address when SMMU is enabled

[valmiki]

> On Sat, May 12, 2018 at 06:25:13PM +0530, valmiki wrote:
>> Hi All,
>>
>> What is the difference between IOVA address and bus address
>> when SMMU is enabled ?
>>
>> Is IOVA address term used only when hypervisor is present ?
>
> IOVA = IO virtual address.  IOVA is the term normally used to describe
> the address used on the _device_ side of an IOMMU.
>
> For any general setup:
>
> RAM ----- MMU ----- DEVICE
>       ^         ^
>   physical   virtual
>   address    address
>
> where "device" can be an IO device or a CPU, the terms still apply.
>
> If you have something like this:
>
> RAM ----- PCI bridge ----- MMU ----- DEVICE
>       ^                ^         ^
>    physical           bus     virtual
>    address          address   address
>
> You could also have (eg, in the case of a system MMU):
>
> RAM ----- MMU ----- PCI bridge ----- DEVICE
>       ^         ^                ^
>    physical  virtual            bus
>    address   address          address
>            (this can also be
>             considered a bus
>             address!)
>
> In both of the above two cases, the PCI bridge may perform some address
> translation, meaning that the bus address is different from the address
> seen on the other side of the bridge.
>
> So, the terms used depend exactly on the overall bus topology.
>
> In the case of a system MMU, where the system MMU sits between peripheral
> devices and RAM, then the bus addresses are the same as the
> _IOVA of the system MMU_.
>
Thanks Russell.

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus