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

[Conor Dooley]

On Mon, Dec 26, 2022 at 05:01:29PM +0100, Andrew Jones wrote:
> 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.

Right, thanks for the explanation. I'm sorry to be a bore here, but should
it be reworded then to "When optimising for low memory consumption on
platforms without SVNAPOT, or if HUGETLBFS is enabled, say N here"?

I don't really care either way to be perfectly honest, so as I said in
my other reply "don't bother resubmitting unless someone else asks for
meaningful changes" ;)

Conor.

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