| Message ID | 20230714165508.94561-5-charlie@rivosinc.com |
|---|---|
| State | New |
| Headers | show |
| Series | RISC-V: mm: Make SV48 the default address space | expand |
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 > >
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. 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.
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(+)