Re: [PATCH] riscv: Support non-coherency memory model

[Gary Guo]

On 23/04/2019 16:46, Guo Ren wrote:
> On Tue, Apr 23, 2019 at 07:55:48AM +0200, Christoph Hellwig wrote:
>> On Tue, Apr 23, 2019 at 08:13:48AM +0800, Guo Ren wrote:
>>>> We should probably start a working group for this ASAP unless we can
>>>> get another working group to help taking care of it.
>>> Good news, I prefer to use instructions directly instead of SBI_CALL.
>>>
>>> Our instruction is "dcache.c/iva %0" (one cache line) and the parameter is
>>> virtual address in S-state. When get into M-state by SBI_CALL, we could
>>> let dcache.c/iva use physical addres directly and it needn't kmap page
>>> for RV32 with highmem (Of cause highmem is not ready in RV32 now).
>>
>> So you only have one instruction variant?  Normally we'd have two or
>> three to implement the non-coherent DMA (or pmem) semantics:
> dcache.c/iva means three instructions:
>   - dcache.cva %0  : writeback     by virtual address cacheline
>   - dcache.iva %0  : invalid       by virtual address cacheline
>   - dcache.civa %0 : writeback+inv by virtual address cacheline
> 
> We also have memory barrier instructions, ref:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/csky/include/asm/barrier.h
> 
>>
>> cache writeback, cache invalidate and potentially cache writeback +
>> invalidate to optimize that case.  Here is the table how Linux
>> uses them for DMA:
>>
>>            |   map          ==  for_device     |   unmap     ==  for_cpu
>>            |----------------------------------------------------------------
>>   TO_DEV   |   writeback        writeback      |   none          none
>>   FROM_DEV |   invalidate       invalidate     |   invalidate*   invalidate*
>>   BIDI     |   writeback+inv    writeback+inv  |   invalidate    invalidate
>>
>>       [*] needed for CPU speculative prefetches
>>
>>
>> We already have a discussion on isa-dev on something like these
>> instructions:
>>
>> https://groups.google.com/a/groups.riscv.org/forum/#!msg/isa-dev/qXbzqaQbDXU/4ThcEAeCCAAJ
>>
>> It got a little side tracked, both due to the usual noise on isa-dev
>> and due to the proposal including a lot more instructions that might be
>> a little more contentious, but it might be a good start to bring this
>> into a working group.
> I think using sbi_call as a temporary solution is a good choice before
> the instruction is determined.
> 
>>
>>>> Also is this really a coherent flag, or an 'uncached' flag like in
>>>> many other architectures?
>>> There are a lot of features about coherency attributes, eg: cacheable,
>>> bufferable, strong order ..., and coherency is a more abstract name to
>>> contain all of these. In our hardware, coherence = uncached +
>>> unbufferable + (stong order).
>>>
>>> But I'm not very care about the name is, uncached is also ok. My key
>>> point is the bits of page attributes is very precious and this patch
>>> will use the last unused attribute bit in PTE.
>>
>> I don't care about the name actually, more about having defined semantics.
>> Totally uncached should include unbuffered.  I don't think we need the
>> strong ordering for DMA memory either.
> Yes, memory don't need strong order and strong order in PMA also implies
> uncached, unbuffered for IO address. If the entry of PMA is strong
> order, the page mapping must be uncached + unbuffered + strong-order
> without _PAGE_COHENCY in PTE.
> 
>>
>>> Another point is we could get more attribute bits by modify the riscv
>>> spec:
>>>   - Remove Global bit, I think it's duplicate with the User bit in linux.
>>
>> It is in Linux, but it is conceptually very different.
> Yes, but hardware could ignore one of them and in riscv linux
> _PAGE_GLOBAL is no use at all, see:
> grep _PAGE_GLOBAL arch/riscv -r
> 
> In fact, the _PAGE_KERNEL for pte doesn't contain _PAGE_GLOBAL and it
> works on FU540 and qemu. As I've mentioned page attribute bits is very
> precious, define a useless bit make people confused.
 >

The fact that it isn't used yet doesn't imply it is not useful. We don't 
use ASIDs at the moment, and without using ASIDs the "global" bit is 
indeed not useful. However with ASIDs the bit will be vital for saving 
TLB spaces. Without the global bit, the kernel pages become synonyms to 
themselves (i.e. they have different tags in TLB but refer to the same 
physical page).

The global bit also exists in many other ISAs as well. It's definitely 
not a "useless" bits.

Moreover, this bit is already implemented in both Rocket and Ariane. It 
is also in the spec for quite a while. The fact that Linux doesn't use 
it at the moment is not a reason for removing it.

> 
>>
>>>   - Change _PAGE_PFN_SHIFT from 10 to 12, because the huge pfn in RV32 is
>>>     very useless and current RV32 linux doesn't even implement highmem.
>>
>> This would seem sensible to me, but I'm not sure everyone agrees.  Even
>> then we are very late in the game for changes like that.
> Agree.
> 
> Best Regards
>   Guo Ren
> 
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