On Thu, Jul 20, 2023 at 08:59:12AM +0200, Alexandre Ghiti wrote:
On Fri, Jul 14, 2023 at 6:56 PM Charlie Jenkins charlie@rivosinc.com wrote:
The behavior of mmap is modified with this patch series, so explain the changes to the mmap hint address behavior.
Signed-off-by: Charlie Jenkins charlie@rivosinc.com
Documentation/riscv/vm-layout.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+)
diff --git a/Documentation/riscv/vm-layout.rst b/Documentation/riscv/vm-layout.rst index 5462c84f4723..892412b91300 100644 --- a/Documentation/riscv/vm-layout.rst +++ b/Documentation/riscv/vm-layout.rst @@ -133,3 +133,25 @@ RISC-V Linux Kernel SV57 ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel __________________|____________|__________________|_________|____________________________________________________________
+Userspace VAs +-------------------- +To maintain compatibility with software that relies on the VA space with a +maximum of 48 bits the kernel will, by default, return virtual addresses to +userspace from a 48-bit range (sv48). This default behavior is achieved by +passing 0 into the hint address parameter of mmap. On CPUs with an address space +smaller than sv48, the CPU maximum supported address space will be the default.
+Software can "opt-in" to receiving VAs from another VA space by providing +a hint address to mmap. A call to mmap is guaranteed to return an address +that will not override the unset left-aligned bits in the hint address, +unless there is no space left in the address space. If there is no space +available in the requested address space, an address in the next smallest +available address space will be returned.
+For example, in order to obtain 48-bit VA space, a hint address greater than +:code:`1 << 38` must be provided.
Is this correct? Shouldn't the hint be strictly greater than the address space it targets? In patch 1, you state that "A hint address passed to mmap will cause the largest address space that fits entirely into the hint to be used", it seems contradictory to me.
That is a mistake, it should have a hint address greater than 1 << 47. I will fix up the wording here.
Note that this is 38 due to sv39 userspace +ending at :code:`1 << 38` and the addresses beyond this are reserved for the +kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
+than or equal to :code:`1 << 47` must be provided.
2.41.0