[PATCH v11 1/3] riscv: mm: modify pte format for Svnapot

[Andrew Jones]

On Mon, Dec 19, 2022 at 09:51:29PM +0000, Conor Dooley wrote:
> On Fri, Dec 16, 2022 at 02:21:07PM +0800, panqinglin2020 at iscas.ac.cn wrote:
> > From: Qinglin Pan <panqinglin2020 at iscas.ac.cn>
> > 
> > Add one alternative to enable/disable svnapot support, enable this static
> > key when "svnapot" is in the "riscv,isa" field of fdt and SVNAPOT compile
> > option is set. It will influence the behavior of has_svnapot. All code
> > dependent on svnapot should make sure that has_svnapot return true firstly.
> > 
> > Modify PTE definition for Svnapot, and creates some functions in pgtable.h
> > to mark a PTE as napot and check if it is a Svnapot PTE. Until now, only
> > 64KB napot size is supported in spec, so some macros has only 64KB version.
> > 
> > Signed-off-by: Qinglin Pan <panqinglin2020 at iscas.ac.cn>
> > Reviewed-by: Andrew Jones <ajones at ventanamicro.com>
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index e1a9fa47f012..25c230e3bf61 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -397,6 +397,25 @@ config RISCV_ISA_C
> >  
> >  	  If you don't know what to do here, say Y.
> >  
> > +config RISCV_ISA_SVNAPOT
> > +	bool "SVNAPOT extension support"
> > +	depends on 64BIT && MMU
> > +	select RISCV_ALTERNATIVE
> > +	default y
> > +	help
> > +	  Allow kernel to detect the SVNAPOT ISA-extension dynamically at boot
> > +	  time and enable its usage.
> > +
> > +	  The SVNAPOT extension is used to mark contiguous PTEs as a range
> > +	  of contiguous virtual-to-physical translations for a naturally
> > +	  aligned power-of-2 (NAPOT) granularity larger than the base 4KB page
> > +	  size. When HUGETLBFS is also selected this option unconditionally
> > +	  allocates some memory for each NAPOT page size supported by the kernel.
> > +	  When optimizing for low memory consumption and for platforms without
> 
> nit: Does this make more sense as "When optimising for low memory
> consumption on platforms without the SVNAPOT extension"...?
> Or does disabling Svnapot save memory on Svnapot capable systems too?

Depends on how we define "save". Disabling RISCV_ISA_SVNAPOT on HUGETLBFS
enabled systems will reduce memory consumption whether the svnapot
extension is present or not. When the extension is present, the memory
consumed may serve a purpose, whereas, on systems without svnapot, the
memory consumed cannot serve a purpose.

Thanks,
drew