Re: [PATCH v3] RISC-V: Fix Maximum Physical Memory 2GiB option for 64bit systems

[Palmer Dabbelt]

On Thu, 04 Apr 2019 23:01:11 PDT (-0700), anup@xxxxxxxxxxxxxx wrote:
> On Fri, Apr 5, 2019 at 11:24 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > On Fri, Apr 05, 2019 at 05:49:34AM +0000, Anup Patel wrote:
> > > The Maximum Physical Memory 2GiB option for 64bit systems is currently
> > > broken because kernel hangs at boot-time when this option is enabled
> > > and the underlying system has more than 2GiB memory.
> > >
> > > This issue can be easily reproduced on SiFive Unleashed board where
> > > we have 8GiB of memory.
> > >
> > > This patch fixes above issue by removing unusable memory region in
> > > setup_bootmem().
> > >
> > > Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> >
> > Btw, what is the rationale behind even offering the 2GiB option and
> > the medlow model on 64-bit?  Do we reall have use cases where the
> > slightly more effient generated code matters so much to keep up
> > the support burden of this mostly unused and unusual configuration?

I'd be OK dropping medlow on rv64.  The performance impact of medany is pretty minor 
these days, and even back when we added the option it was mostly paranoia about 
the linker being correct.  medany has been solid on rv64 for a while, but we 
did recently find a pretty major toolchain bug in medany/rv32 so if we were to 
drop that we'd need to enforce a pretty recent binutils version (the latest 
release is OK, I'd have to check on the previous one).  I'd prefer to avoid 
that, and since medany doesn't really buy us anything on rv32 it seems fine to 
leave it that way.

Given how many issues we've found here recently I doubt anyone is flipping this 
option away from the default for their base ISA.

> Yes, if we want to use medlow with 64bit then max physical memory
> can be 2GiB. Otherwise, I don't any practical use of restricting max
> physical memory to 2GiB on 64bit systems.
>
> BTW, as-per latest revision of RISC-V priviledge spece the MMU SV32
> mode will be able to access 34bit physical memory (i.e. 16GiB) so
> max physical memory 2GiB will also be required for 32bit system for
> medlow.

It's a bit more complicated that than.  The 34-bit physical addresses have been 
in the spec for a long time, but we've never bothered supporting them in Linux.  
Right now we assume we can map all physical memory into a contiguous region, 
which is why we tie PAGE_OFFSET to the maximum physical memory.  It's 
impossible to map all physical memory with 34-bit physical addresses on a 
32-bit system, so we'll need to break that assumption to move forward with 
34-bit physical addresses anyway.

Additionally: medlow can map everything on 32-bit systems anyway, so moving to 
medany doesn't buy you anything.