Physical address returned by IODMACommand exceeds hw.memsize

29 Nov 2010

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Hi,

consider the following code:

void* allocPage(uint64_t* phys_addr)
{
        void* result = NULL;
        IOBufferMemoryDescriptor* descriptor =  NULL;

        descriptor = IOBufferMemoryDescriptor::withOptions(

kIODirectionInOut|kIOMemoryPhysicallyContiguous,

                                          page_size, page_size);

        descriptor->prepare();

        *phys_addr = -1;

        IODMACommand* dma =
IODMACommand::withSpecification(kIODMACommandOutputHost64,

                                      64,

                                      0,

                                      IODMACommand::kMapped,

                                      0,

                                      1);

         if (kIOReturnSuccess == dma->setMemoryDescriptor(descriptor))
         {
                 IODMACommand::Segment64 segments[1];
                 UInt32  numSegments = 1;
                 UInt64  offset = 0;

                 if (kIOReturnSuccess ==
dma->gen64IOVMSegments(&offset, segments, &numSegments))
                         *phys_addr = segments[0].fIOVMAddr;

                 dma->clearMemoryDescriptor();
         }

         dma->release();

         result = descriptor->getBytesNoCopy();
         IOLog("virtual address 0x%016llx(%llu) is mapped to physical
address 0x%016llx(%lld)\n", result, result, *phys_addr, *phys_addr);
         return result;
}

On a MacPro4,1 with OSX 10.6.4, 6gb of ram installed (hw.memsize:
6442450944) and boot-args="debug=0x14e arch=x86_64 himemory_mode=2
dart=0":
virtual address 0xffffff8087c35000(18446743526231461888) is mapped to
physical address 0x00000001f202b000(8355229696)
virtual address 0xffffff8087c3e000(18446743526231498752) is mapped to
physical address 0x00000001f22ac000(8357855232)
virtual address 0xffffff8087c45000(18446743526231527424) is mapped to
physical address 0x00000001f1d2d000(8352092160)
virtual address 0xffffff8087c46000(18446743526231531520) is mapped to
physical address 0x00000001f1eae000(8353669120)

Well, everything works just fine, so I'm just curious, where does this
extra memory come from?

Thanks in advance.
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Physical address returned by IODMACommand exceeds hw.memsize

Maxim Zhuravlev