dma_alloc_coherent - cma - and IOMMU question

dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

I've posted the following email to vger.kernel.org but got no response. I am
trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
Here is what I posted to LKML

"start quote"

Sorry for the noise. I've read everything DMA in the kernel Doc dir and
searched the web to no avail. So I thought I might get some useful info here.

I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
currently doing 8MB DMAs to and from our device using the in kernel CMA
"cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
capable so the in kernel CMA has been great and replaced our old bigphysarea
usage.

We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
returned from the dma_alloc_coherent function to our device as the "bus/pci"
address to use.

We also use remap_pfn_range on that dma_addr_t *dma_handle returned from 
the dma_alloc_coherent function to mmap userland to the buffer. All is good
until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
quietly never completes or the system gets borked.

[  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
domain=0x001b address=0x00000000aa500000 flags=0x0010]
[  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
domain=0x001b address=0x00000000aa500040 flags=0x0010]

Here are the IOMMU settings in my kernel config:

#grep IOMMU .config
# CONFIG_GART_IOMMU is not set
# CONFIG_CALGARY_IOMMU is not set
CONFIG_IOMMU_HELPER=y
CONFIG_VFIO_IOMMU_TYPE1=m
CONFIG_IOMMU_API=y
CONFIG_IOMMU_SUPPORT=y
CONFIG_AMD_IOMMU=y
# CONFIG_AMD_IOMMU_STATS is not set
CONFIG_AMD_IOMMU_V2=m
CONFIG_INTEL_IOMMU=y
CONFIG_INTEL_IOMMU_DEFAULT_ON=y
CONFIG_INTEL_IOMMU_FLOPPY_WA=y
# CONFIG_IOMMU_STRESS is not set


>From reading the in kernel doc it would appear that we could in fact, using
the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
our device could DMA anywhere, even above the 4GB address space limit of our
device. But before going through this larger change to our GPL driver, I
want to understand if and/or why the dma_alloc_coherent function does not
appear to set up the IOMMU for me. Is the IOMMU only supported for
"streaming" DMA type and not for "coherent"? I read no reference to this in
the kernel doc?

Any hints would be greatly appreciated. Again, sorry for the noise. 


"end quote"

Sorry if this is not correct place to get info on the AMD IOMMU support in
the kernel. If it's not could someone point me in the right direction?

Thanks and Regards
Mark

Re: dma_alloc_coherent - cma - and IOMMU question

[Alex Williamson]

On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
> I've posted the following email to vger.kernel.org but got no response. I am
> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
> Here is what I posted to LKML
> 
> "start quote"
> 
> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
> searched the web to no avail. So I thought I might get some useful info here.
> 
> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
> currently doing 8MB DMAs to and from our device using the in kernel CMA
> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
> capable so the in kernel CMA has been great and replaced our old bigphysarea
> usage.
> 
> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
> returned from the dma_alloc_coherent function to our device as the "bus/pci"
> address to use.
> 
> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from 
> the dma_alloc_coherent function to mmap userland to the buffer. All is good
> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
> quietly never completes or the system gets borked.

The dma_addr_t is an I/O virtual address (IOVA), it's the address the
*device* uses to access the buffer returned by dma_alloc_coherent.  If
you mmap that address through /dev/mem, you're getting the processor
view of the address, which is not IOMMU translated.  Only the device
uses the dma_addr_t, processor accesses need to use the returned void*,
or some sort of virt_to_phys() version of that to allow userspace to
mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
virt_to_bus() translation of the void* buffer, so the code happens to
work, but is still and incorrect usage of the DMA API.

BTW, depending on how much if your driver is in userspace, vfio might be
a better choice for device access and IOMMU programming.  Thanks,

Alex

> [  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> domain=0x001b address=0x00000000aa500000 flags=0x0010]
> [  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> domain=0x001b address=0x00000000aa500040 flags=0x0010]
> 
> Here are the IOMMU settings in my kernel config:
> 
> #grep IOMMU .config
> # CONFIG_GART_IOMMU is not set
> # CONFIG_CALGARY_IOMMU is not set
> CONFIG_IOMMU_HELPER=y
> CONFIG_VFIO_IOMMU_TYPE1=m
> CONFIG_IOMMU_API=y
> CONFIG_IOMMU_SUPPORT=y
> CONFIG_AMD_IOMMU=y
> # CONFIG_AMD_IOMMU_STATS is not set
> CONFIG_AMD_IOMMU_V2=m
> CONFIG_INTEL_IOMMU=y
> CONFIG_INTEL_IOMMU_DEFAULT_ON=y
> CONFIG_INTEL_IOMMU_FLOPPY_WA=y
> # CONFIG_IOMMU_STRESS is not set
> 
> 
> From reading the in kernel doc it would appear that we could in fact, using
> the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
> our device could DMA anywhere, even above the 4GB address space limit of our
> device. But before going through this larger change to our GPL driver, I
> want to understand if and/or why the dma_alloc_coherent function does not
> appear to set up the IOMMU for me. Is the IOMMU only supported for
> "streaming" DMA type and not for "coherent"? I read no reference to this in
> the kernel doc?
> 
> Any hints would be greatly appreciated. Again, sorry for the noise. 
> 
> 
> "end quote"
> 
> Sorry if this is not correct place to get info on the AMD IOMMU support in
> the kernel. If it's not could someone point me in the right direction?
> 
> Thanks and Regards
> Mark
> 
> 
> 
> _______________________________________________
> iommu mailing list
> iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

Re: dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

On 01/30/2015 03:11 PM, Alex Williamson wrote:
> On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
>> I've posted the following email to vger.kernel.org but got no response. I am
>> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
>> Here is what I posted to LKML
>>
>> "start quote"
>>
>> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
>> searched the web to no avail. So I thought I might get some useful info here.
>>
>> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
>> currently doing 8MB DMAs to and from our device using the in kernel CMA
>> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
>> capable so the in kernel CMA has been great and replaced our old bigphysarea
>> usage.
>>
>> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
>> returned from the dma_alloc_coherent function to our device as the "bus/pci"
>> address to use.
>>
>> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
>> the dma_alloc_coherent function to mmap userland to the buffer. All is good
>> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
>> quietly never completes or the system gets borked.
> 
> The dma_addr_t is an I/O virtual address (IOVA), it's the address the
> *device* uses to access the buffer returned by dma_alloc_coherent.  If
> you mmap that address through /dev/mem, you're getting the processor
> view of the address, which is not IOMMU translated.  Only the device
> uses the dma_addr_t, processor accesses need to use the returned void*,
> or some sort of virt_to_phys() version of that to allow userspace to
> mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
> virt_to_bus() translation of the void* buffer, so the code happens to
> work, but is still and incorrect usage of the DMA API.
> 

Thanks Alex,

Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
virt_to_bus() translation of the void* buffer?
 
This is what I am doing. Returning dma_usr_addr to userland.

dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);

remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
                              size, vma->vm_page_prot); 

So what is incorrect/wrong here. I just checked and even with IOMMU enabled
dma_pci_addr ==  virt_to_bus(dma_usr_addr)

And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
as long as I figure out what I'm doing wrong?

Thanks Alex, and please set me straight.

Regards
Mark


> BTW, depending on how much if your driver is in userspace, vfio might be
> a better choice for device access and IOMMU programming.  Thanks,
> 
> Alex
> 
>> [  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
>> domain=0x001b address=0x00000000aa500000 flags=0x0010]
>> [  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
>> domain=0x001b address=0x00000000aa500040 flags=0x0010]
>>
>> Here are the IOMMU settings in my kernel config:
>>
>> #grep IOMMU .config
>> # CONFIG_GART_IOMMU is not set
>> # CONFIG_CALGARY_IOMMU is not set
>> CONFIG_IOMMU_HELPER=y
>> CONFIG_VFIO_IOMMU_TYPE1=m
>> CONFIG_IOMMU_API=y
>> CONFIG_IOMMU_SUPPORT=y
>> CONFIG_AMD_IOMMU=y
>> # CONFIG_AMD_IOMMU_STATS is not set
>> CONFIG_AMD_IOMMU_V2=m
>> CONFIG_INTEL_IOMMU=y
>> CONFIG_INTEL_IOMMU_DEFAULT_ON=y
>> CONFIG_INTEL_IOMMU_FLOPPY_WA=y
>> # CONFIG_IOMMU_STRESS is not set
>>
>>
>>  From reading the in kernel doc it would appear that we could in fact, using
>> the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
>> our device could DMA anywhere, even above the 4GB address space limit of our
>> device. But before going through this larger change to our GPL driver, I
>> want to understand if and/or why the dma_alloc_coherent function does not
>> appear to set up the IOMMU for me. Is the IOMMU only supported for
>> "streaming" DMA type and not for "coherent"? I read no reference to this in
>> the kernel doc?
>>
>> Any hints would be greatly appreciated. Again, sorry for the noise.
>>
>>
>> "end quote"
>>
>> Sorry if this is not correct place to get info on the AMD IOMMU support in
>> the kernel. If it's not could someone point me in the right direction?
>>
>> Thanks and Regards
>> Mark
>>
>>
>>
>> _______________________________________________
>> iommu mailing list
>> iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> 
> 
> 
> 

Re: dma_alloc_coherent - cma - and IOMMU question

[Alex Williamson]

On Fri, 2015-01-30 at 16:07 -0500, Mark Hounschell wrote:
> On 01/30/2015 03:11 PM, Alex Williamson wrote:
> > On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
> >> I've posted the following email to vger.kernel.org but got no response. I am
> >> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
> >> Here is what I posted to LKML
> >>
> >> "start quote"
> >>
> >> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
> >> searched the web to no avail. So I thought I might get some useful info here.
> >>
> >> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
> >> currently doing 8MB DMAs to and from our device using the in kernel CMA
> >> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
> >> capable so the in kernel CMA has been great and replaced our old bigphysarea
> >> usage.
> >>
> >> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
> >> returned from the dma_alloc_coherent function to our device as the "bus/pci"
> >> address to use.
> >>
> >> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
> >> the dma_alloc_coherent function to mmap userland to the buffer. All is good
> >> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
> >> quietly never completes or the system gets borked.
> > 
> > The dma_addr_t is an I/O virtual address (IOVA), it's the address the
> > *device* uses to access the buffer returned by dma_alloc_coherent.  If
> > you mmap that address through /dev/mem, you're getting the processor
> > view of the address, which is not IOMMU translated.  Only the device
> > uses the dma_addr_t, processor accesses need to use the returned void*,
> > or some sort of virt_to_phys() version of that to allow userspace to
> > mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
> > virt_to_bus() translation of the void* buffer, so the code happens to
> > work, but is still and incorrect usage of the DMA API.
> > 
> 
> Thanks Alex,
> 
> Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
> virt_to_bus() translation of the void* buffer?

Yes
 
> This is what I am doing. Returning dma_usr_addr to userland.
> 
> dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);
> 
> remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
>                               size, vma->vm_page_prot); 
> 
> So what is incorrect/wrong here. I just checked and even with IOMMU enabled
> dma_pci_addr ==  virt_to_bus(dma_usr_addr)

You're passing NULL to dma_alloc_coherent as the device.  That's
completely invalid when a real IOMMU is present.  When you do that, you
take a code path in amd_iommu that simply allocates a buffer and returns
__pa() of that buffer as the DMA address.  So the IOMMU isn't programmed
for the device AND userspace is mapping the wrong range.  This explains
the page faults below.  You need to to also use dma_user_addr in place
of dma_pci_addr in the remap_pfn_range.

> And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
> as long as I figure out what I'm doing wrong?

If you pass an actual device to dma_alloc_coherent, then the IOMMU
should be programmed correctly.  I don't know how CMA fits into your
picture since dma_alloc_coherent allocates a buffer independent of CMA.
Wouldn't you need to allocate the buffer from the CMA pool and then call
dma_map_page() on it in order to use CMA?  Thanks,

Alex

> > BTW, depending on how much if your driver is in userspace, vfio might be
> > a better choice for device access and IOMMU programming.  Thanks,
> > 
> > Alex
> > 
> >> [  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> >> domain=0x001b address=0x00000000aa500000 flags=0x0010]
> >> [  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> >> domain=0x001b address=0x00000000aa500040 flags=0x0010]
> >>
> >> Here are the IOMMU settings in my kernel config:
> >>
> >> #grep IOMMU .config
> >> # CONFIG_GART_IOMMU is not set
> >> # CONFIG_CALGARY_IOMMU is not set
> >> CONFIG_IOMMU_HELPER=y
> >> CONFIG_VFIO_IOMMU_TYPE1=m
> >> CONFIG_IOMMU_API=y
> >> CONFIG_IOMMU_SUPPORT=y
> >> CONFIG_AMD_IOMMU=y
> >> # CONFIG_AMD_IOMMU_STATS is not set
> >> CONFIG_AMD_IOMMU_V2=m
> >> CONFIG_INTEL_IOMMU=y
> >> CONFIG_INTEL_IOMMU_DEFAULT_ON=y
> >> CONFIG_INTEL_IOMMU_FLOPPY_WA=y
> >> # CONFIG_IOMMU_STRESS is not set
> >>
> >>
> >>  From reading the in kernel doc it would appear that we could in fact, using
> >> the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
> >> our device could DMA anywhere, even above the 4GB address space limit of our
> >> device. But before going through this larger change to our GPL driver, I
> >> want to understand if and/or why the dma_alloc_coherent function does not
> >> appear to set up the IOMMU for me. Is the IOMMU only supported for
> >> "streaming" DMA type and not for "coherent"? I read no reference to this in
> >> the kernel doc?
> >>
> >> Any hints would be greatly appreciated. Again, sorry for the noise.
> >>
> >>
> >> "end quote"
> >>
> >> Sorry if this is not correct place to get info on the AMD IOMMU support in
> >> the kernel. If it's not could someone point me in the right direction?
> >>
> >> Thanks and Regards
> >> Mark
> >>
> >>
> >>
> >> _______________________________________________
> >> iommu mailing list
> >> iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> >> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> > 
> > 
> > 
> > 
> 

Re: dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

On 01/30/2015 04:51 PM, Alex Williamson wrote:
> On Fri, 2015-01-30 at 16:07 -0500, Mark Hounschell wrote:
>> On 01/30/2015 03:11 PM, Alex Williamson wrote:
>>> On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
>>>> I've posted the following email to vger.kernel.org but got no response. I am
>>>> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
>>>> Here is what I posted to LKML
>>>>
>>>> "start quote"
>>>>
>>>> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
>>>> searched the web to no avail. So I thought I might get some useful info here.
>>>>
>>>> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
>>>> currently doing 8MB DMAs to and from our device using the in kernel CMA
>>>> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
>>>> capable so the in kernel CMA has been great and replaced our old bigphysarea
>>>> usage.
>>>>
>>>> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
>>>> returned from the dma_alloc_coherent function to our device as the "bus/pci"
>>>> address to use.
>>>>
>>>> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
>>>> the dma_alloc_coherent function to mmap userland to the buffer. All is good
>>>> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
>>>> quietly never completes or the system gets borked.
>>>
>>> The dma_addr_t is an I/O virtual address (IOVA), it's the address the
>>> *device* uses to access the buffer returned by dma_alloc_coherent.  If
>>> you mmap that address through /dev/mem, you're getting the processor
>>> view of the address, which is not IOMMU translated.  Only the device
>>> uses the dma_addr_t, processor accesses need to use the returned void*,
>>> or some sort of virt_to_phys() version of that to allow userspace to
>>> mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
>>> virt_to_bus() translation of the void* buffer, so the code happens to
>>> work, but is still and incorrect usage of the DMA API.
>>>
>>
>> Thanks Alex,
>>
>> Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
>> virt_to_bus() translation of the void* buffer?
>
> Yes
>
>> This is what I am doing. Returning dma_usr_addr to userland.
>>
>> dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);
>>
>> remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
>>                                size, vma->vm_page_prot);
>>
>> So what is incorrect/wrong here. I just checked and even with IOMMU enabled
>> dma_pci_addr ==  virt_to_bus(dma_usr_addr)
>
> You're passing NULL to dma_alloc_coherent as the device.  That's
> completely invalid when a real IOMMU is present.  When you do that, you
> take a code path in amd_iommu that simply allocates a buffer and returns
> __pa() of that buffer as the DMA address.  So the IOMMU isn't programmed
> for the device AND userspace is mapping the wrong range.  This explains
> the page faults below.  You need to to also use dma_user_addr in place
> of dma_pci_addr in the remap_pfn_range.
>
>> And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
>> as long as I figure out what I'm doing wrong?
>
> If you pass an actual device to dma_alloc_coherent, then the IOMMU
> should be programmed correctly.  I don't know how CMA fits into your
> picture since dma_alloc_coherent allocates a buffer independent of CMA.
> Wouldn't you need to allocate the buffer from the CMA pool and then call
> dma_map_page() on it in order to use CMA?  Thanks,
>

Thanks for that Alex.

 From what I understand of CMA, and it seems provable to me, is that 
dma_alloc_coherent allocates my 8MB buffer from CMA defined on the 
cmdline. Without CMA specified on the cmdline, dma_alloc_coherent 
definitely fails to allocate an 8MB contiguous buffer. From what I've 
read about it, it is supposed to transparently "just work" when 
dma_alloc_coherent is used?

However. when I pass an actual device (device_eprm) to 
dma_alloc_coherent that was obtained in the following code:

if (alloc_chrdev_region(&eprm_major, 0, 1, EPRM_NAME) != 0) {
         return -ENODEV;
}

eprm_cdevice = cdev_alloc();
if (eprm_cdevice <= 0) {
         return -ENODEV;
}

eprm_cdevice->owner = THIS_MODULE;
cdev_init(eprm_cdevice, &eprm_fops);
if (cdev_add(eprm_cdevice, eprm_major, 1) < 0) {
         return -ENODEV;
}

class_eprm = class_create(THIS_MODULE, "eprm");
if (IS_ERR(class_eprm)) {
         return -ENODEV;
}

device_eprm = device_create(class_eprm, NULL, eprm_major, NULL, "eprm");
if (IS_ERR(device_eprm)) {
         return -ENODEV;
}

then dma_alloc_coherent returns 0?

Regards
Mark

> Alex
>
>>> BTW, depending on how much if your driver is in userspace, vfio might be
>>> a better choice for device access and IOMMU programming.  Thanks,
>>>
>>> Alex
>>>
>>>> [  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
>>>> domain=0x001b address=0x00000000aa500000 flags=0x0010]
>>>> [  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
>>>> domain=0x001b address=0x00000000aa500040 flags=0x0010]
>>>>
>>>> Here are the IOMMU settings in my kernel config:
>>>>
>>>> #grep IOMMU .config
>>>> # CONFIG_GART_IOMMU is not set
>>>> # CONFIG_CALGARY_IOMMU is not set
>>>> CONFIG_IOMMU_HELPER=y
>>>> CONFIG_VFIO_IOMMU_TYPE1=m
>>>> CONFIG_IOMMU_API=y
>>>> CONFIG_IOMMU_SUPPORT=y
>>>> CONFIG_AMD_IOMMU=y
>>>> # CONFIG_AMD_IOMMU_STATS is not set
>>>> CONFIG_AMD_IOMMU_V2=m
>>>> CONFIG_INTEL_IOMMU=y
>>>> CONFIG_INTEL_IOMMU_DEFAULT_ON=y
>>>> CONFIG_INTEL_IOMMU_FLOPPY_WA=y
>>>> # CONFIG_IOMMU_STRESS is not set
>>>>
>>>>
>>>>   From reading the in kernel doc it would appear that we could in fact, using
>>>> the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
>>>> our device could DMA anywhere, even above the 4GB address space limit of our
>>>> device. But before going through this larger change to our GPL driver, I
>>>> want to understand if and/or why the dma_alloc_coherent function does not
>>>> appear to set up the IOMMU for me. Is the IOMMU only supported for
>>>> "streaming" DMA type and not for "coherent"? I read no reference to this in
>>>> the kernel doc?
>>>>
>>>> Any hints would be greatly appreciated. Again, sorry for the noise.
>>>>
>>>>
>>>> "end quote"
>>>>
>>>> Sorry if this is not correct place to get info on the AMD IOMMU support in
>>>> the kernel. If it's not could someone point me in the right direction?
>>>>
>>>> Thanks and Regards
>>>> Mark
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> iommu mailing list
>>>> iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
>>>
>>>
>>>
>>>
>>
>
>
>
>

Re: dma_alloc_coherent - cma - and IOMMU question

[Alex Williamson]

[cc +joerg]

On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
> On 01/30/2015 04:51 PM, Alex Williamson wrote:
> > On Fri, 2015-01-30 at 16:07 -0500, Mark Hounschell wrote:
> >> On 01/30/2015 03:11 PM, Alex Williamson wrote:
> >>> On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
> >>>> I've posted the following email to vger.kernel.org but got no response. I am
> >>>> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
> >>>> Here is what I posted to LKML
> >>>>
> >>>> "start quote"
> >>>>
> >>>> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
> >>>> searched the web to no avail. So I thought I might get some useful info here.
> >>>>
> >>>> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
> >>>> currently doing 8MB DMAs to and from our device using the in kernel CMA
> >>>> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
> >>>> capable so the in kernel CMA has been great and replaced our old bigphysarea
> >>>> usage.
> >>>>
> >>>> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
> >>>> returned from the dma_alloc_coherent function to our device as the "bus/pci"
> >>>> address to use.
> >>>>
> >>>> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
> >>>> the dma_alloc_coherent function to mmap userland to the buffer. All is good
> >>>> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
> >>>> quietly never completes or the system gets borked.
> >>>
> >>> The dma_addr_t is an I/O virtual address (IOVA), it's the address the
> >>> *device* uses to access the buffer returned by dma_alloc_coherent.  If
> >>> you mmap that address through /dev/mem, you're getting the processor
> >>> view of the address, which is not IOMMU translated.  Only the device
> >>> uses the dma_addr_t, processor accesses need to use the returned void*,
> >>> or some sort of virt_to_phys() version of that to allow userspace to
> >>> mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
> >>> virt_to_bus() translation of the void* buffer, so the code happens to
> >>> work, but is still and incorrect usage of the DMA API.
> >>>
> >>
> >> Thanks Alex,
> >>
> >> Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
> >> virt_to_bus() translation of the void* buffer?
> >
> > Yes
> >
> >> This is what I am doing. Returning dma_usr_addr to userland.
> >>
> >> dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);
> >>
> >> remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
> >>                                size, vma->vm_page_prot);
> >>
> >> So what is incorrect/wrong here. I just checked and even with IOMMU enabled
> >> dma_pci_addr ==  virt_to_bus(dma_usr_addr)
> >
> > You're passing NULL to dma_alloc_coherent as the device.  That's
> > completely invalid when a real IOMMU is present.  When you do that, you
> > take a code path in amd_iommu that simply allocates a buffer and returns
> > __pa() of that buffer as the DMA address.  So the IOMMU isn't programmed
> > for the device AND userspace is mapping the wrong range.  This explains
> > the page faults below.  You need to to also use dma_user_addr in place
> > of dma_pci_addr in the remap_pfn_range.
> >
> >> And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
> >> as long as I figure out what I'm doing wrong?
> >
> > If you pass an actual device to dma_alloc_coherent, then the IOMMU
> > should be programmed correctly.  I don't know how CMA fits into your
> > picture since dma_alloc_coherent allocates a buffer independent of CMA.
> > Wouldn't you need to allocate the buffer from the CMA pool and then call
> > dma_map_page() on it in order to use CMA?  Thanks,
> >
> 
> Thanks for that Alex.
> 
>  From what I understand of CMA, and it seems provable to me, is that 
> dma_alloc_coherent allocates my 8MB buffer from CMA defined on the 
> cmdline. Without CMA specified on the cmdline, dma_alloc_coherent 
> definitely fails to allocate an 8MB contiguous buffer. From what I've 
> read about it, it is supposed to transparently "just work" when 
> dma_alloc_coherent is used?

Yes, if you're running with the software iotlb (aka. bounce buffers),
then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
dma_generic_alloc_coherent(), which attempts to use CMA via
dma_alloc_from_contiguous().

If you look at the same path with AMD-Vi, dma_ops->alloc is
amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
allocate the buffer.  I don't see any CMA integration along that path.
If you were using Intel VT-d, then the buffer is again allocated with
dma_alloc_from_contiguous() and should use CMA.  This was added in
kernel v3.16, but no corresponding AMD-Vi change was added.  Joerg, this
might be an easily fixed oversight.

> However. when I pass an actual device (device_eprm) to 
> dma_alloc_coherent that was obtained in the following code:
> 
> if (alloc_chrdev_region(&eprm_major, 0, 1, EPRM_NAME) != 0) {
>          return -ENODEV;
> }
> 
> eprm_cdevice = cdev_alloc();
> if (eprm_cdevice <= 0) {
>          return -ENODEV;
> }
> 
> eprm_cdevice->owner = THIS_MODULE;
> cdev_init(eprm_cdevice, &eprm_fops);
> if (cdev_add(eprm_cdevice, eprm_major, 1) < 0) {
>          return -ENODEV;
> }
> 
> class_eprm = class_create(THIS_MODULE, "eprm");
> if (IS_ERR(class_eprm)) {
>          return -ENODEV;
> }
> 
> device_eprm = device_create(class_eprm, NULL, eprm_major, NULL, "eprm");
> if (IS_ERR(device_eprm)) {
>          return -ENODEV;
> }
> 
> then dma_alloc_coherent returns 0?

Ugh, creating a virtual device is no better than passing a NULL device.
Some piece of hardware out there is doing DMA, that's the device that
needs to be associated with the dma_alloc_coherent call.  An arbitrary
char device is useless.  Thanks,

Alex

> >>> BTW, depending on how much if your driver is in userspace, vfio might be
> >>> a better choice for device access and IOMMU programming.  Thanks,
> >>>
> >>> Alex 
> >>>
> >>>> [  106.115725] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> >>>> domain=0x001b address=0x00000000aa500000 flags=0x0010]
> >>>> [  106.115729] AMD-Vi: Event logged [IO_PAGE_FAULT device=03:00.0
> >>>> domain=0x001b address=0x00000000aa500040 flags=0x0010]
> >>>>
> >>>> Here are the IOMMU settings in my kernel config:
> >>>>
> >>>> #grep IOMMU .config
> >>>> # CONFIG_GART_IOMMU is not set
> >>>> # CONFIG_CALGARY_IOMMU is not set
> >>>> CONFIG_IOMMU_HELPER=y
> >>>> CONFIG_VFIO_IOMMU_TYPE1=m
> >>>> CONFIG_IOMMU_API=y
> >>>> CONFIG_IOMMU_SUPPORT=y
> >>>> CONFIG_AMD_IOMMU=y
> >>>> # CONFIG_AMD_IOMMU_STATS is not set
> >>>> CONFIG_AMD_IOMMU_V2=m
> >>>> CONFIG_INTEL_IOMMU=y
> >>>> CONFIG_INTEL_IOMMU_DEFAULT_ON=y
> >>>> CONFIG_INTEL_IOMMU_FLOPPY_WA=y
> >>>> # CONFIG_IOMMU_STRESS is not set
> >>>>
> >>>>
> >>>>   From reading the in kernel doc it would appear that we could in fact, using
> >>>> the IOMMU and the dma_map_sg function, get rid of the CMA requirement and
> >>>> our device could DMA anywhere, even above the 4GB address space limit of our
> >>>> device. But before going through this larger change to our GPL driver, I
> >>>> want to understand if and/or why the dma_alloc_coherent function does not
> >>>> appear to set up the IOMMU for me. Is the IOMMU only supported for
> >>>> "streaming" DMA type and not for "coherent"? I read no reference to this in
> >>>> the kernel doc?
> >>>>
> >>>> Any hints would be greatly appreciated. Again, sorry for the noise.
> >>>>
> >>>>
> >>>> "end quote"
> >>>>
> >>>> Sorry if this is not correct place to get info on the AMD IOMMU support in
> >>>> the kernel. If it's not could someone point me in the right direction?
> >>>>
> >>>> Thanks and Regards
> >>>> Mark
> >>>>
> >>>>
> >>>>
> >>>> _______________________________________________
> >>>> iommu mailing list
> >>>> iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> >>>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
> >
> >
> 

Re: dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

On 02/02/2015 12:35 PM, Alex Williamson wrote:
> [cc +joerg]
>
> On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
>> On 01/30/2015 04:51 PM, Alex Williamson wrote:
>>> On Fri, 2015-01-30 at 16:07 -0500, Mark Hounschell wrote:
>>>> On 01/30/2015 03:11 PM, Alex Williamson wrote:
>>>>> On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
>>>>>> I've posted the following email to vger.kernel.org but got no response. I am
>>>>>> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
>>>>>> Here is what I posted to LKML
>>>>>>
>>>>>> "start quote"
>>>>>>
>>>>>> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
>>>>>> searched the web to no avail. So I thought I might get some useful info here.
>>>>>>
>>>>>> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
>>>>>> currently doing 8MB DMAs to and from our device using the in kernel CMA
>>>>>> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
>>>>>> capable so the in kernel CMA has been great and replaced our old bigphysarea
>>>>>> usage.
>>>>>>
>>>>>> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
>>>>>> returned from the dma_alloc_coherent function to our device as the "bus/pci"
>>>>>> address to use.
>>>>>>
>>>>>> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
>>>>>> the dma_alloc_coherent function to mmap userland to the buffer. All is good
>>>>>> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
>>>>>> quietly never completes or the system gets borked.
>>>>>
>>>>> The dma_addr_t is an I/O virtual address (IOVA), it's the address the
>>>>> *device* uses to access the buffer returned by dma_alloc_coherent.  If
>>>>> you mmap that address through /dev/mem, you're getting the processor
>>>>> view of the address, which is not IOMMU translated.  Only the device
>>>>> uses the dma_addr_t, processor accesses need to use the returned void*,
>>>>> or some sort of virt_to_phys() version of that to allow userspace to
>>>>> mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
>>>>> virt_to_bus() translation of the void* buffer, so the code happens to
>>>>> work, but is still and incorrect usage of the DMA API.
>>>>>
>>>>
>>>> Thanks Alex,
>>>>
>>>> Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
>>>> virt_to_bus() translation of the void* buffer?
>>>
>>> Yes
>>>
>>>> This is what I am doing. Returning dma_usr_addr to userland.
>>>>
>>>> dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);
>>>>
>>>> remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
>>>>                                 size, vma->vm_page_prot);
>>>>
>>>> So what is incorrect/wrong here. I just checked and even with IOMMU enabled
>>>> dma_pci_addr ==  virt_to_bus(dma_usr_addr)
>>>
>>> You're passing NULL to dma_alloc_coherent as the device.  That's
>>> completely invalid when a real IOMMU is present.  When you do that, you
>>> take a code path in amd_iommu that simply allocates a buffer and returns
>>> __pa() of that buffer as the DMA address.  So the IOMMU isn't programmed
>>> for the device AND userspace is mapping the wrong range.  This explains
>>> the page faults below.  You need to to also use dma_user_addr in place
>>> of dma_pci_addr in the remap_pfn_range.
>>>

For the userland mmap of the buffer, I originally was using

remap_pfn_range(vma, vma->vm_start, (long) virt_to_bus(dma_user_addr),
                                 (uint64_t)sc->s_dma_io_size, 
vma->vm_page_prot);

but then mistakenly changed it to use dma_pci_addr when I thought that
dma_pci_addr ==  virt_to_bus(dma_usr_addr). My bad. I keep reading that
virt_to_bus and friends are going away so thought, again mistakenly, 
this was how I could get away with not using it. I'm back to using 
virt_to_bus(dma_user_addr).

>>>> And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
>>>> as long as I figure out what I'm doing wrong?
>>>
>>> If you pass an actual device to dma_alloc_coherent, then the IOMMU
>>> should be programmed correctly.  I don't know how CMA fits into your
>>> picture since dma_alloc_coherent allocates a buffer independent of CMA.
>>> Wouldn't you need to allocate the buffer from the CMA pool and then call
>>> dma_map_page() on it in order to use CMA?  Thanks,
>>>
>>
>> Thanks for that Alex.
>>
>>   From what I understand of CMA, and it seems provable to me, is that
>> dma_alloc_coherent allocates my 8MB buffer from CMA defined on the
>> cmdline. Without CMA specified on the cmdline, dma_alloc_coherent
>> definitely fails to allocate an 8MB contiguous buffer. From what I've
>> read about it, it is supposed to transparently "just work" when
>> dma_alloc_coherent is used?
>
> Yes, if you're running with the software iotlb (aka. bounce buffers),
> then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
> dma_generic_alloc_coherent(), which attempts to use CMA via
> dma_alloc_from_contiguous().
>
> If you look at the same path with AMD-Vi, dma_ops->alloc is
> amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
> allocate the buffer.  I don't see any CMA integration along that path.
> If you were using Intel VT-d, then the buffer is again allocated with
> dma_alloc_from_contiguous() and should use CMA.  This was added in
> kernel v3.16, but no corresponding AMD-Vi change was added.  Joerg, this
> might be an easily fixed oversight.
>
>> However. when I pass an actual device (device_eprm) to
>> dma_alloc_coherent that was obtained in the following code:
>>
>> if (alloc_chrdev_region(&eprm_major, 0, 1, EPRM_NAME) != 0) {
>>           return -ENODEV;
>> }
>>
>> eprm_cdevice = cdev_alloc();
>> if (eprm_cdevice <= 0) {
>>           return -ENODEV;
>> }
>>
>> eprm_cdevice->owner = THIS_MODULE;
>> cdev_init(eprm_cdevice, &eprm_fops);
>> if (cdev_add(eprm_cdevice, eprm_major, 1) < 0) {
>>           return -ENODEV;
>> }
>>
>> class_eprm = class_create(THIS_MODULE, "eprm");
>> if (IS_ERR(class_eprm)) {
>>           return -ENODEV;
>> }
>>
>> device_eprm = device_create(class_eprm, NULL, eprm_major, NULL, "eprm");
>> if (IS_ERR(device_eprm)) {
>>           return -ENODEV;
>> }
>>
>> then dma_alloc_coherent returns 0?
>
> Ugh, creating a virtual device is no better than passing a NULL device.
> Some piece of hardware out there is doing DMA, that's the device that
> needs to be associated with the dma_alloc_coherent call.  An arbitrary
> char device is useless.  Thanks,
>

Got it. Using the real "device" works. I am now able to dma with the 
IOMMU enabled. But you are correct about CMA and dma_alloc_coherent not 
playing with each other when using the AMD IOMMU. With the IOMMU enabled 
I cannot get any where near 8MB of contiguous memory using 
dma_alloc_coherent. Ouch!

So thank you for setting me straight concerning this DMA and IOMMU 
thing. I would be more than happy to test any easily fixed oversights.

Regards
Mark

Re: dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

On 02/02/2015 12:35 PM, Alex Williamson wrote:
> [cc +joerg]
>
> On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
>> On 01/30/2015 04:51 PM, Alex Williamson wrote:
>>> On Fri, 2015-01-30 at 16:07 -0500, Mark Hounschell wrote:
>>>> On 01/30/2015 03:11 PM, Alex Williamson wrote:
>>>>> On Fri, 2015-01-30 at 19:12 +0000, Mark Hounschell wrote:
>>>>>> I've posted the following email to vger.kernel.org but got no response. I am
>>>>>> trying to adapt some of our out of kernel GPL drivers to use the AMD IOMMU.
>>>>>> Here is what I posted to LKML
>>>>>>
>>>>>> "start quote"
>>>>>>
>>>>>> Sorry for the noise. I've read everything DMA in the kernel Doc dir and
>>>>>> searched the web to no avail. So I thought I might get some useful info here.
>>>>>>
>>>>>> I'm currently using a 3.18.3 (x86_64) kernel on an AMD platform. I am
>>>>>> currently doing 8MB DMAs to and from our device using the in kernel CMA
>>>>>> "cma=64M@0-4G" with no problems. This device is not DAC or scatter/gather
>>>>>> capable so the in kernel CMA has been great and replaced our old bigphysarea
>>>>>> usage.
>>>>>>
>>>>>> We simply use dma_alloc_coherent and pass the dma_addr_t *dma_handle
>>>>>> returned from the dma_alloc_coherent function to our device as the "bus/pci"
>>>>>> address to use.
>>>>>>
>>>>>> We also use remap_pfn_range on that dma_addr_t *dma_handle returned from
>>>>>> the dma_alloc_coherent function to mmap userland to the buffer. All is good
>>>>>> until I enable the IOMMU. I then either get IO_PAGE_FAULTs, the DMA just
>>>>>> quietly never completes or the system gets borked.
>>>>>
>>>>> The dma_addr_t is an I/O virtual address (IOVA), it's the address the
>>>>> *device* uses to access the buffer returned by dma_alloc_coherent.  If
>>>>> you mmap that address through /dev/mem, you're getting the processor
>>>>> view of the address, which is not IOMMU translated.  Only the device
>>>>> uses the dma_addr_t, processor accesses need to use the returned void*,
>>>>> or some sort of virt_to_phys() version of that to allow userspace to
>>>>> mmap it through devmem.  Without an IOMMU, the dma_addr_t is simply a
>>>>> virt_to_bus() translation of the void* buffer, so the code happens to
>>>>> work, but is still and incorrect usage of the DMA API.
>>>>>
>>>>
>>>> Thanks Alex,
>>>>
>>>> Are you saying the WITH an IOMMU that dma_addr_t is NOT simply a
>>>> virt_to_bus() translation of the void* buffer?
>>>
>>> Yes
>>>
>>>> This is what I am doing. Returning dma_usr_addr to userland.
>>>>
>>>> dma_usr_addr = (char *)dma_alloc_coherent(NULL, size, dma_pci_addr, GFP_KERNEL);
>>>>
>>>> remap_pfn_range(vma, vma->vm_start, dma_pci_addr >> PAGE_SHIFT,
>>>>                                 size, vma->vm_page_prot);
>>>>
>>>> So what is incorrect/wrong here. I just checked and even with IOMMU enabled
>>>> dma_pci_addr ==  virt_to_bus(dma_usr_addr)
>>>
>>> You're passing NULL to dma_alloc_coherent as the device.  That's
>>> completely invalid when a real IOMMU is present.  When you do that, you
>>> take a code path in amd_iommu that simply allocates a buffer and returns
>>> __pa() of that buffer as the DMA address.  So the IOMMU isn't programmed
>>> for the device AND userspace is mapping the wrong range.  This explains
>>> the page faults below.  You need to to also use dma_user_addr in place
>>> of dma_pci_addr in the remap_pfn_range.
>>>
>>>> And can I assume that support is there for the IOMMU , CMA, and dma_alloc_coherent
>>>> as long as I figure out what I'm doing wrong?
>>>
>>> If you pass an actual device to dma_alloc_coherent, then the IOMMU
>>> should be programmed correctly.  I don't know how CMA fits into your
>>> picture since dma_alloc_coherent allocates a buffer independent of CMA.
>>> Wouldn't you need to allocate the buffer from the CMA pool and then call
>>> dma_map_page() on it in order to use CMA?  Thanks,
>>>
>>
>> Thanks for that Alex.
>>
>>   From what I understand of CMA, and it seems provable to me, is that
>> dma_alloc_coherent allocates my 8MB buffer from CMA defined on the
>> cmdline. Without CMA specified on the cmdline, dma_alloc_coherent
>> definitely fails to allocate an 8MB contiguous buffer. From what I've
>> read about it, it is supposed to transparently "just work" when
>> dma_alloc_coherent is used?
>
> Yes, if you're running with the software iotlb (aka. bounce buffers),
> then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
> dma_generic_alloc_coherent(), which attempts to use CMA via
> dma_alloc_from_contiguous().
>
> If you look at the same path with AMD-Vi, dma_ops->alloc is
> amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
> allocate the buffer.  I don't see any CMA integration along that path.
> If you were using Intel VT-d, then the buffer is again allocated with
> dma_alloc_from_contiguous() and should use CMA.  This was added in
> kernel v3.16, but no corresponding AMD-Vi change was added.  Joerg, this
> might be an easily fixed oversight.
>

Hi Alex,

Has this possibly been fixed in 3.19?

Thanks
Mark

Re: dma_alloc_coherent - cma - and IOMMU question

[Joerg Roedel]

On Mon, Feb 02, 2015 at 10:35:36AM -0700, Alex Williamson wrote:
> [cc +joerg]
> 
> On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
> >  From what I understand of CMA, and it seems provable to me, is that 
> > dma_alloc_coherent allocates my 8MB buffer from CMA defined on the 
> > cmdline. Without CMA specified on the cmdline, dma_alloc_coherent 
> > definitely fails to allocate an 8MB contiguous buffer. From what I've 
> > read about it, it is supposed to transparently "just work" when 
> > dma_alloc_coherent is used?
> 
> Yes, if you're running with the software iotlb (aka. bounce buffers),
> then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
> dma_generic_alloc_coherent(), which attempts to use CMA via
> dma_alloc_from_contiguous().
> 
> If you look at the same path with AMD-Vi, dma_ops->alloc is
> amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
> allocate the buffer.  I don't see any CMA integration along that path.
> If you were using Intel VT-d, then the buffer is again allocated with
> dma_alloc_from_contiguous() and should use CMA.  This was added in
> kernel v3.16, but no corresponding AMD-Vi change was added.  Joerg, this
> might be an easily fixed oversight.

Right, CMA is not yet integrated in the AMD IOMMU driver, nobody asked
fot it until now :) I will add that fall-back there for big allocations.


	Joerg

Re: dma_alloc_coherent - cma - and IOMMU question

[Mark Hounschell]

On 02/04/2015 09:15 AM, Joerg Roedel wrote:
> On Mon, Feb 02, 2015 at 10:35:36AM -0700, Alex Williamson wrote:
>> [cc +joerg]
>>
>> On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
>>>   From what I understand of CMA, and it seems provable to me, is that
>>> dma_alloc_coherent allocates my 8MB buffer from CMA defined on the
>>> cmdline. Without CMA specified on the cmdline, dma_alloc_coherent
>>> definitely fails to allocate an 8MB contiguous buffer. From what I've
>>> read about it, it is supposed to transparently "just work" when
>>> dma_alloc_coherent is used?
>>
>> Yes, if you're running with the software iotlb (aka. bounce buffers),
>> then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
>> dma_generic_alloc_coherent(), which attempts to use CMA via
>> dma_alloc_from_contiguous().
>>
>> If you look at the same path with AMD-Vi, dma_ops->alloc is
>> amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
>> allocate the buffer.  I don't see any CMA integration along that path.
>> If you were using Intel VT-d, then the buffer is again allocated with
>> dma_alloc_from_contiguous() and should use CMA.  This was added in
>> kernel v3.16, but no corresponding AMD-Vi change was added.  Joerg, this
>> might be an easily fixed oversight.
>
> Right, CMA is not yet integrated in the AMD IOMMU driver, nobody asked
> fot it until now :) I will add that fall-back there for big allocations.
>
>
> 	Joerg
>
>

Hi Joerg,

Is device to device (crossing pci busses) or even pci-e to pci-e support 
there or in the future?

Thanks and Regards
Mark