On Wed, Apr 24, 2019 at 12:45:56PM +0000, Gary Guo wrote: > The RISC-V privileged spec is explicitly designed to allow the > techniques described above (this is the sole purpose of MSTATUS.TVM). It > might be as high performance as a hardware with H-extension, but is > definitely a legit use case. In fact, it is vital for use cases like > recursive virtualization. > > Also, I believe the PTE format of RISC-V is already frozen -- therefore > it is impossible now to merge GLOBAL and USER bit, nor to replace RSW > bit with another bit.
Yes, I do not think we can just repurpose a bit. Even using a currently unused one would require some gymnastics. That being said IFF we want to support non-coherent DMA (and I think we do as people glue together their SOCs using shoestring and paper clips, as already demonstrated by Andes and C-SKY in RISC-V space, and most arm, mips and ppc SOCs) we need something like this flag. The current RISC-V method that only allows M-mode to set up such attributes on a small number or PMP regions just doesn't work well with the way how Linux and most non-trivial OSes implement DMA memory allocations. Note that I said well - in theory we can have a firmware provided uncached pool - that is what Linux does on most nommu (that is without pagetables) ports, but the fixed sized pool really does suck and will make users very unhappy.
