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

[Mike Rapoport <rppt@linux.ibm.com>]

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@linux.ibm.com> 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@linux.ibm.com> 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.

> Regards,
> Anup
> 

-- 
Sincerely yours,
Mike.


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