Mario,
Yes. It is part of the regular DRAM that can be used as standard Linux memory or memory for the networking device. If you have 16 GB of DRAM memory you decide at boot time how much of it should be used by Linux and how much should be used by the networking device. It is all coherent.
Magnus
Den 6 nov 2014 20:28 skrev "Mario Smarduch" <m.smarduch@xxxxxxxxxxx>:
Is the memory you're trying to pass into guest to communicate
with your network devices coherently part of onboard DRAM?
- Mario
On 11/06/2014 05:51 AM, Magnus Karlsson wrote:
> Hi,
>
> I have a problem with the cacheability memory attribute permissions that
> are inserted into the stage 2 page table by KVM for ARM. Would be great
> if I could get some advise. Maybe I am doing something wrong in the set
> up or this kind of device has not been encountered before.
>
> So here we go. I have a board with an LSI Axxia AXM5516 SoC used for
> networking equipment. In addition to 16 A15 cores, it contains a lot of
> networking HW. The networking HW communicates with the cores through the
> use of a part of the regular cache coherent DRAM memory. This region has
> to be consecutive and is today carved out using a memreserve in the
> device tree. Usually this region is several times larger than the amount
> of memory given to Linux.
>
> My problem starts when I would like to access this region from a guest.
> I have made a direct mapping of this region into the guest by adding a
> sysbus_mmio region into the machine model of Qemu (which will end up
> calling the KVM_SET_USER_MEMORY_REGION ioctl). And yes I will start to
> use VFIO as soon as it is accepted ;-). When the stage 2 translation
> entry for this region is about to get entered in
> arch/arm/kvm/mmu.c:user_mem_abort() I get to this line:
>
> if (kvm_is_mmio_pfn(pfn))
> mem_type = PAGE_S2_DEVICE;
>
>
> bool kvm_is_mmio_pfn(pfn_t pfn)
> {
> if (pfn_valid(pfn))
> return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
>
> return true;
> }
>
> For the device to work I need mem_type to continue to be PAGE_S2. But
> kvm_is_mmio_pfn() returns true as PageReserved() is true due to my
> memreserved region. I get an uncached device memory mapping as stage 2
> overrides the mapping in stage 1 (and the cores do not see what the
> networking hardware wrote in the caches). If I instead remove the device
> memory completely from Linux, then pfn_valid() will be false and
> kvm_is_mmi_pfn() will still return true (but I would like all memory to
> be part of Linux, so not an idea I fancy). So what to do? Do I need to
> register the region differently with KVM (and Qemu), should I use the
> VFIO patches instead because they solve the problem, or does this code
> assume that all reserved pages are non-coherent device memory and it
> needs to be extended? Removing the code is of course not a solution even
> for me as there are plenty of other more normal devices on the SoC that
> are non-coherent.
>
> BTW, I cannot use the CMA (instead of memreserve) since it does not work
> with highmem.
>
> Thanks: Magnus
>
>
> --
>
> *Magnus Karlsson*
>
> Software Development Engineering Manager
>
> Avago Technologies (formerly LSI Logic)
>
> Box 1024, Knarrarnäsgatan 15
>
> SE-164 21 Kista, Sweden
>
> TEL +46 8 594 607 09
>
> FAX +46 8 594 607 10
>
> CELL +46 73 80 444 88
>
> magnus.karlsson@xxxxxxxxxxxxx <mailto:magnus.karlsson@xxxxxxxxxxxxx>
>
>
>
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