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

[Alex Ghiti <alex@ghiti.fr>]

Hi Atish,

Le 6/10/20 à 2:32 PM, Atish Patra a écrit :
> On Wed, Jun 3, 2020 at 8:36 AM Alexandre Ghiti <alex@ghiti.fr> 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?

Yes, a 1GB hugepage will cover those 2MB: I rely on the previous boot 
stage to mark this region
as reserved if there is something there (like opensbi). Otherwise, the 
kernel will indeed try to
allocate memory from there :)

Alex


>> 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
>>
>>
>