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
