Re: [PATCH v2] RISC-V: Always compile mm/init.c with cmodel=medany

From: Anup Patel <anup@brainfault.org>
To: Palmer Dabbelt <palmer@sifive.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Alan Kao <alankao@andestech.com>, Anup Patel <Anup.Patel@wdc.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	"linux-kernel@vger.kernel.org List"
	<linux-kernel@vger.kernel.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Atish Patra <Atish.Patra@wdc.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH v2] RISC-V: Always compile mm/init.c with cmodel=medany
Date: Tue, 26 Mar 2019 09:31:26 +0530	[thread overview]
Message-ID: <CAAhSdy2POcL2Y8VzULCHRa6cLKZ82GkEgXCgx_SoNZ1vMPvQjQ@mail.gmail.com> (raw)
In-Reply-To: <mhng-8e00a3e3-5a69-4828-b7bb-2bf000199b48@palmer-si-x1c4>

On Tue, Mar 26, 2019 at 7:52 AM Palmer Dabbelt <palmer@sifive.com> wrote:
>
> On Mon, 25 Mar 2019 00:01:45 PDT (-0700), anup@brainfault.org wrote:
> > On Mon, Mar 25, 2019 at 12:18 PM Christoph Hellwig <hch@infradead.org> wrote:
> >>
> >> On Mon, Mar 25, 2019 at 01:25:50PM +0800, Alan Kao wrote:
> >> > Hi Anup,
> >> >
> >> > Sorry for being late to the party.  I think one more thing should
> >> > move together with setup_vm():
> >>
> >> Ah, I wonded about that yesterday but wasn't sure.  Maybe notrace
> >> is a little cleaner?  Either way we should probably document both
> >> the mcmodel and notrace assumptions in source comments for the next
> >> person touching this code.
> >
> > The setup_vm() should be allowed to call other functions within mm/init.c
> > so let's go with file-level notrace (just like how it was done) for
> > kernel/setup.c
> >
> > I certainly add comments for setup_vm() based on all our findings so far.
>
> Sorry for being slow here, but this is the right approach: setup_vm is called
> before relocate, which means the page tables won't be set up correctly for
> absolute addressing.  We instead build setup_vm with medany, which causes all
> addressing to be PC-relative.  This is all a bit of a hack, but it's the only
> way we have to do this right now.
>
> You should be able to add a preprocessor #error to check the code model with
> something like this
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index b379a75ac6a6..d6fde6af8d75 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -172,6 +172,9 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
>         }
>  }
>
> +#ifndef __riscv_cmodel_medany
> +#error "setup_vm() is called from head.S before relocate and must not make any absolute references."
> +#endif
>  asmlinkage void __init setup_vm(void)
>  {
>         extern char _start;
>
> Marking this notrace is the right thing to do, as it can't call into any
> functions that aren't medany (there's probably other issues as well, since this
> is so early).

Thanks for the suggestion, I will add "#error" in v4 of this patch.

>
> Sorry I missed this the first time around, I wasn't paying enough attention.
>
> Can someone add instructions for 32-bit boots to the QEMU wiki?  It sounds like
> it's time to add that to the testing list...

To start with, I have send a patch for adding rv32_defconfig which is right now
"defconfig" plus "CONFIG_ARCH_RV32I=y"

Regards,
Anup