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

[Guo Ren]

Hi Gary,

On Tue, Apr 23, 2019 at 03:57:30PM +0000, Gary Guo wrote:
> >>> 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.
> 

Look:
linux-next git:(riscv_asid_allocator_v2)$ grep GLOBAL arch/riscv -r
arch/riscv/include/asm/pgtable-bits.h:#define _PAGE_GLOBAL    (1 << 5)    /* Global */
arch/riscv/include/asm/pgtable-bits.h:                                    _PAGE_USER | _PAGE_GLOBAL))

Your patch tell us _PAGE_USER and _PAGE_GLOBAL are duplicate and why we
couldn't make _PAGE_USER implies _PAGE_GLOBAL? Can you give an example
of a real scene in PTE about:
  _PAGE_USER:0 + _PAGE_GLOBAL:1
or
  _PAGE_USER:1 + _PAGE_GLOBAL:0

Of cause I know USER & GLOBAL are conceptually very different, but
there are only 10 attribute-bits for riscv (In fact we've wasted two bits
to support huge RV32-pfn :P). So I think it is time to merge these two bits
before hardware supports GLOBAL. Reserve them for future!

Best Regards
 Guo Ren