RE: 32bit kernel is broken for Linux-5.1-rc1 due to GCC cmodel=medlow

From: Gary Guo <gary@garyguo.net>
To: Anup Patel <anup@brainfault.org>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>,
	Palmer Dabbelt <palmer@sifive.com>,
	Anup Patel <anup.patel@wdc.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Atish Patra <atish.patra@wdc.com>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>
Subject: RE: 32bit kernel is broken for Linux-5.1-rc1 due to GCC cmodel=medlow
Date: Sun, 24 Mar 2019 01:56:22 +0000	[thread overview]
Message-ID: <LO2P265MB0847CF97521DEF5AB0EA7B21D65D0@LO2P265MB0847.GBRP265.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <CAAhSdy07GsHXpG9MJZ4aiB5uG7D+9RA+17Nb1MrWdFg9ee6rUA@mail.gmail.com>

I think what he means is to force the file containing setup_vm to be compiled using medany, even if cmodel is configured to be medlow.

Regards,
Gary

> -----Original Message-----
> From: linux-riscv <linux-riscv-bounces@lists.infradead.org> On Behalf Of Anup
> Patel
> Sent: Sunday, March 24, 2019 01:42
> To: Christoph Hellwig <hch@infradead.org>
> Cc: Damien Le Moal <Damien.LeMoal@wdc.com>; Palmer Dabbelt
> <palmer@sifive.com>; Anup Patel <anup.patel@wdc.com>; Mike Rapoport
> <rppt@linux.ibm.com>; Atish Patra <atish.patra@wdc.com>; linux-
> riscv@lists.infradead.org
> Subject: Re: 32bit kernel is broken for Linux-5.1-rc1 due to GCC cmodel=medlow
> 
> On Sat, Mar 23, 2019 at 10:51 PM Christoph Hellwig <hch@infradead.org>
> wrote:
> >
> > On Fri, Mar 22, 2019 at 07:15:37PM +0530, Anup Patel wrote:
> > > >
> > > > I am not saying the revert is the final solution.  I'm just saying
> > > > that 5.1-rc is not the place we should be doing the major surgery.
> > > > Revert fox 5.1, fix for real for 5.2+.
> > >
> > > If we revert for time being then even fixmap support has to be
> > > reverted.
> > >
> > > The patch only updates access to kernel symbols via __load_pa() and
> > > __load_va(). There is no functional change as such. The patch is
> > > tested on QEMU/virt 64bit, QEMU/virt 32bit, and SiFive Unleashed.
> >
> > Just curios - if you just move setup_vm and dependencies into a new
> > file that is compiled with -mcmodel=medany, just like setup.c, what
> > issues would we still have that require explicitly passing the load
> > address?
> 
> There are four cases here:
> 1. 64bit with cmodel=medany (default for 64bit system) 2. 64bit with
> cmodel=medlow 3. 32bit with cmodel=medany 4. 32bit with cmodel=medlow
> (default for 32bit system)
> 
> I had tried using cmodel=medany as default for both 64bit and 32bit system and
> it works fine because there is no issue with cmodel=medany but it is a very crude
> fix and things break again if someone explicitly selects cmodel=medlow using
> kconfig.
> 
> The patch for using __load_va()/__load_pa() accessor in setup_vm() solves the
> problem for all four cases above.
> 
> If you still think the patch is too big as rcX fix then alternate approach is:
> 1. For Linux-5.1-rcX, we use cmodel=medany by default for both 32bit and 64bit
> RISC-V (I can send a patch for this immediately) 2. Consider
> __load_va()/__load_pa() patch for Linux-5.2
> 
> Regards,
> Anup
> 
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