Re: [PATCH 3/3] RISC-V: Allow booting kernel from any 4KB aligned address

[Anup Patel]

On Fri, Mar 15, 2019 at 9:28 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 14, 2019 at 11:28:32PM +0530, Anup Patel wrote:
> > On Thu, Mar 14, 2019 at 12:23 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Mar 14, 2019 at 02:36:01AM +0530, Anup Patel wrote:
> > > > On Thu, Mar 14, 2019 at 12:01 AM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Mar 12, 2019 at 10:08:22PM +0000, Anup Patel wrote:
> > > > > > Currently, we have to boot RISCV64 kernel from a 2MB aligned physical
> > > > > > address and RISCV32 kernel from a 4MB aligned physical address. This
> > > > > > constraint is because initial pagetable setup (i.e. setup_vm()) maps
> > > > > > entire RAM using hugepages (i.e. 2MB for 3-level pagetable and 4MB for
> > > > > > 2-level pagetable).
> > > > > >
> > > > > > Further, the above booting contraint also results in memory wastage
> > > > > > because if we boot kernel from some <xyz> address (which is not same as
> > > > > > RAM start address) then RISCV kernel will map PAGE_OFFSET virtual address
> > > > > > lineraly to <xyz> physical address and memory between RAM start and <xyz>
> > > > > > will be reserved/unusable.
> > > > > >
> > > > > > For example, RISCV64 kernel booted from 0x80200000 will waste 2MB of RAM
> > > > > > and RISCV32 kernel booted from 0x80400000 will waste 4MB of RAM.
> > > > > >
> > > > > > This patch re-writes the initial pagetable setup code to allow booting
> > > > > > RISV32 and RISCV64 kernel from any 4KB (i.e. PAGE_SIZE) aligned address.
> > > > > >
> > > > > > To achieve this:
> > > > > > 1. We map kernel, dtb and only some amount of RAM (few MBs) using 4KB
> > > > > >    mappings in setup_vm() (called from head.S)
> > > > > > 2. Once we reach paging_init() (called from setup_arch()) after
> > > > > >    memblock setup, we map all available memory banks using 4KB
> > > > > >    mappings and memblock APIs.
> > > > >
> > > > > I'm not really familiar with RISC-V, but my guess would be that you'd get
> > > > > worse TLB performance with 4KB mappings. Not mentioning the amount of
> > > > > memory required for the page table itself.
> > > >
> > > > I agree we will see a hit in TLB performance due to 4KB mappings.
> > > >
> > > > To address this we can create, 2MB (or 4MB on 32bit system) mappings
> > > > whenever load_pa is aligned to it otherwise we prefer 4KB mappings. In other
> > > > words, we create bigger mappings whenever possible and fallback to 4KB
> > > > mappings when not possible.
> > > >
> > > > This way if kernel is booted from 2MB (or 4MB) aligned address then we will
> > > > see good TLB performance for kernel addresses. Also, users are still free to
> > > > boot Linux RISC-V kernel from any 4KB aligned address.
> > > >
> > > > Of course, we will have to document this as part of Linux RISC-V booting
> > > > requirements under Documentation/ (which does not exist currently).
> > > >
> > > > >
> > > > > If the only goal is to utilize the physical memory below the kernel, it
> > > > > simply should not be reserved at the first place, something like:
> > > >
> > > > Well, our goal was two-fold:
> > > >
> > > > 1. We wanted to unify boot-time alignment requirements for 32bit and
> > > > 64bit RISC-V systems
> > >
> > > Can't they both start from 4MB aligned address provided the memory below
> > > the kernel can be freed?
> >
> > Yes, they can both start from 4MB aligned address.
> >
> > >
> > > > 2. Save memory by allowing users to place kernel just after the runtime
> > > > firmware at starting of RAM.
> > >
> > > If the firmware should be alive after kernel boot, it's memory is the only
> > > part that should be reserved below the kernel. Otherwise, the entire region
> > > <physical memory start> - <kernel start> can be free.
> > >
> > > Using 4K pages for the swapper_pg_dir is quite a change and I'm not
> > > convinced its really justified.
> >
> > I understand your concern about TLB performance and more page
> > tables.
> >
> > Not just 2MB/4MB mappings, we should be able to create even 1GB
> > mappings as well for good TLB performance.
> >
> > I suggest we should use best possible mapping size (4KB, 2MB, or
> > 1GB) based on alignment of kernel load address. This way users can
> > boot from any 4KB aligned address and setup_vm() will try to use
> > biggest possible mapping size.
> >
> > For example, If the kernel load address is aligned to 2MB then we 2MB
> > mappings bigger mappings and use fewer page tables. Same thing
> > possible for 1GB mappings as well.
>
> I still don't get why it is that important to relax alignment of the kernel
> load address. Provided you can use the memory below the kernel, it really
> should not matter.

The original idea was just to relax the alignment constraint on the kernel
load address.

What I am suggesting now is to improve this patch so that we can
dynamically select mapping size based on kernel load address. This
will achieve both:
1. Relaxed constraint on kernel load address
2. Better TLB performance whenever possible

Regards,
Anup