Re: [PATCH 0/2] PUD/PGDIR entries for linear mapping

[Atish Patra]

On Wed, Jun 3, 2020 at 8:36 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> This small patchset intends to use PUD/PGDIR entries for linear mapping
> in order to better utilize TLB.
>
> At the moment, only PMD entries can be used since on common platforms
> (qemu/unleashed), the kernel is loaded at DRAM + 2MB which dealigns virtual
> and physical addresses and then prevents the use of PUD/PGDIR entries.
> So the kernel must be able to get those 2MB for PAGE_OFFSET to map the
> beginning of the DRAM: this is achieved in patch 1.
>

I don't have in depth knowledge of how mm code works so this question
may be a completely
stupid one :). Just for my understanding,
As per my understanding, kernel will map those 2MB of memory but never use it.
How does the kernel ensure that it doesn't allocate any memory from those 2MB
memory if it is not marked as reserved?

> But furthermore, at the moment, the firmware (opensbi) explicitly asks the
> kernel not to map the region it occupies, which is on those common
> platforms at the very beginning of the DRAM and then it also dealigns
> virtual and physical addresses. I proposed a patch here:
>
> https://github.com/riscv/opensbi/pull/167
>
> that removes this 'constraint' but *not* all the time as it offers some
> kind of protection in case PMP is not available. So sometimes, we may
> have a part of the memory below the kernel that is removed creating a
> misalignment between virtual and physical addresses. So for performance
> reasons, we must at least make sure that PMD entries can be used: that
> is guaranteed by patch 1 too.
>
> Finally the second patch simply improves best_map_size so that whenever
> possible, PUD/PGDIR entries are used.
>
> Below is the kernel page table without this patch on a 6G platform:
>
> ---[ Linear mapping ]---
> 0xffffc00000000000-0xffffc00176e00000    0x0000000080200000 5998M PMD     D A . . . W R V
>
> And with this patchset + opensbi patch:
>
> ---[ Linear mapping ]---
> 0xffffc00000000000-0xffffc00140000000 0x0000000080000000         5G PUD     D A . . . W R V
> 0xffffc00140000000-0xffffc00177000000    0x00000001c0000000 880M PMD     D A . . . W R V
>
> Alexandre Ghiti (2):
>   riscv: Get memory below load_pa while ensuring linear mapping is PMD
>     aligned
>   riscv: Use PUD/PGDIR entries for linear mapping when possible
>
>  arch/riscv/include/asm/page.h |  8 ++++
>  arch/riscv/mm/init.c          | 69 +++++++++++++++++++++++++++++------
>  2 files changed, 65 insertions(+), 12 deletions(-)
>
> --
> 2.20.1
>
>


-- 
Regards,
Atish