Hi Stephen,
the memory in Linux is mapped cached, while in your L4Re side, you
mapped it uncached. Did you try mapping it cached on the L4Re side?
Or you do a cache clean on the str string in Linux.
BR, Adam
On Mon Apr 13, 2026 at 14:53:00 +0800, stephen.yang wrote:
> Hi l4-hackers,
> I am trying to pass a kernel-allocated memory buffer to a virtual device,
> but I am running into an issue. I would appreciate any help or suggestions.
>
>
> 1.Kernel side(driver)
> In my platform driver, I allocate memory using devm_kzalloc, write a string
> into it, and pass its physical address to the device via an MMIO register:
> ¡¡
> void* buf = devm_kzalloc(&pdev->dev, 0x200, GFP_KERNEL);
> if (!buf)
> return -ENOMEM;
> phys_addr_t pa = virt_to_phys(buf);
> char *str = "hi stephen";
> memcpy(buf, str, strlen(str) + 1);
> wmb();
> writeq((u64)pa, hb->base + 0x00);
> ¡¡
>
>
> My understanding is: allocate kernel memory ¡ú write data ¡ú pass physical
> address to the device.
>
>
> 2. Memory layout
> I observed that the allocated physical address falls within the RAM regions
> configured via bootargs : 0x66200000 - 0x6fffffff
> So my assumption was that if the virtual device maps this physical memory
> region, it should be able to access the data written by the kernel.
>
>
> 3. Virtual device side (L4Re)
> On the virtual device side, I map the physical memory using L4Re RM attach:
> void write(unsigned reg, char /*size*/, l4_uint64_t value, unsigned)
> {
> if (reg == 0x00) {
> addr = value;
>
>
> auto d = L4Re::Env::env()->get_cap<L4Re::Dataspace>("lram");
>
>
> l4_addr_t vaddr;
> l4_addr_t base = 0x66200000;
> l4_addr_t size = 0x09E00000;
> int r = L4Re::Env::env()->rm()->attach( &vaddr, size,
> L4Re::Rm::F::Search_addr | L4Re::Rm::F::RW |
> L4Re::Rm::F::Cache_uncached,
> L4::Ipc::make_cap_rw(d),
> 0,
> L4_PAGESHIFT);
> if (r) {
> printf("can't attach hb memory\n");
> return;
> }
> char *dst = reinterpret_cast<char *>(vaddr + (addr - base));
> for (int i = 0; i < 20; i++) {
> printf("%02x ", (unsigned char)dst[i]);
> }
> printf("\n");
> printf("get msg for guest: %s\n", dst);
> }
> }
>
>
> 4.Problem
> However, I cannot read the string written by the kernel. The memory content I
> get appears to be uninitialized or unrelated data.
>
>
> 5.Question
> I can confirm that the lram dataspace capability does correctly correspond to
> the physical memory region 0x66200000 - 0x6fffffff,
> and this region is valid and usable RAM on the system.
>
>
> I am unsure where the issue lies:
> (1) Is this approach (kernel allocation ¡ú pass physical address ¡ú direct
> mapping in virtual device) actually valid?
> (2) Could this be a cache coherence / DMA issue?
> (3) Or is there something incorrect in my L4Re RM attach usage?
>
>
> Any insights would be greatly appreciated.
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