linux-kernel - Re: [PATCH v2 2/5] RISC-V: Make setup_vm() independent of GCC code model

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Date:   Mon, 25 Mar 2019 09:49:55 +0530
From:   Anup Patel <anup@...infault.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <Atish.Patra@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Christoph Hellwig <hch@...radead.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] RISC-V: Make setup_vm() independent of GCC code model

On Sat, Mar 23, 2019 at 9:15 PM Mike Rapoport <rppt@...ux.ibm.com> wrote:
>
> On Thu, Mar 21, 2019 at 09:47:47AM +0000, Anup Patel wrote:
> > The setup_vm() must access kernel symbols in a position independent way
> > because it will be called from head.S with MMU off.
> >
> > If we compile kernel with cmodel=medany then PC-relative addressing will
> > be used in setup_vm() to access kernel symbols so it works perfectly fine.
> >
> > Although, if we compile kernel with cmodel=medlow then either absolute
> > addressing or PC-relative addressing (based on whichever requires fewer
> > instructions) is used to access kernel symbols in setup_vm(). This can
> > break setup_vm() whenever any absolute addressing is used to access
> > kernel symbols.
> >
> > With the movement of setup_vm() from kernel/setup.c to mm/init.c, the
> > setup_vm() is now broken for cmodel=medlow but it works perfectly fine
> > for cmodel=medany.
> >
> > This patch fixes setup_vm() and makes it independent of GCC code model
> > by accessing kernel symbols relative to kernel load address instead of
> > assuming PC-relative addressing.
> >
> > Fixes: 6f1e9e946f0b ("RISC-V: Move setup_vm() to mm/init.c")
> > Signed-off-by: Anup Patel <anup.patel@....com>
> > ---
> >  arch/riscv/kernel/head.S |  1 +
> >  arch/riscv/mm/init.c     | 73 ++++++++++++++++++++++++++--------------
> >  2 files changed, 49 insertions(+), 25 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index fe884cd69abd..7966262b4f9d 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -62,6 +62,7 @@ clear_bss_done:
> >
> >       /* Initialize page tables and relocate to virtual addresses */
> >       la sp, init_thread_union + THREAD_SIZE
> > +     la a0, _start
> >       call setup_vm
> >       call relocate
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index b379a75ac6a6..e38f8195e45b 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -172,55 +172,78 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
> >       }
> >  }
> >
> > -asmlinkage void __init setup_vm(void)
> > +static inline void *__load_addr(void *ptr, uintptr_t load_pa)
> >  {
> >       extern char _start;
> > +     uintptr_t va = (uintptr_t)ptr;
> > +     uintptr_t sz = (uintptr_t)(&_end) - (uintptr_t)(&_start);
> > +
> > +     if (va >= PAGE_OFFSET && va <= (PAGE_OFFSET + sz))
> > +             return (void *)(load_pa + (va - PAGE_OFFSET));
> > +     return (void *)va;
> > +}
> > +
> > +#define __load_va(ptr, load_pa)      __load_addr(ptr, load_pa)
> > +#define __load_pa(ptr, load_pa)      ((uintptr_t)__load_addr(ptr, load_pa))
> > +
> > +asmlinkage void __init setup_vm(uintptr_t load_pa)
> > +{
> >       uintptr_t i;
> > -     uintptr_t pa = (uintptr_t) &_start;
> > +#ifndef __PAGETABLE_PMD_FOLDED
> > +     pmd_t *pmdp;
> > +#endif
> > +     pgd_t *pgdp;
> > +     phys_addr_t map_pa;
> > +     pgprot_t tableprot = __pgprot(_PAGE_TABLE);
> >       pgprot_t prot = __pgprot(pgprot_val(PAGE_KERNEL) | _PAGE_EXEC);
> >
> > -     va_pa_offset = PAGE_OFFSET - pa;
> > -     pfn_base = PFN_DOWN(pa);
> > +     va_pa_offset = PAGE_OFFSET - load_pa;
> > +     pfn_base = PFN_DOWN(load_pa);
> >
> >       /* Sanity check alignment and size */
> >       BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
> > -     BUG_ON((pa % (PAGE_SIZE * PTRS_PER_PTE)) != 0);
> > +     BUG_ON((load_pa % (PAGE_SIZE * PTRS_PER_PTE)) != 0);
> >
> >  #ifndef __PAGETABLE_PMD_FOLDED
> > -     trampoline_pg_dir[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
> > -             pfn_pgd(PFN_DOWN((uintptr_t)trampoline_pmd),
> > -                     __pgprot(_PAGE_TABLE));
> > -     trampoline_pmd[0] = pfn_pmd(PFN_DOWN(pa), prot);
> > +     pgdp = __load_va(trampoline_pg_dir, load_pa);
> > +     map_pa = __load_pa(trampoline_pmd, load_pa);
> > +     pgdp[(PAGE_OFFSET >> PGDIR_SHIFT) % PTRS_PER_PGD] =
>
> Can we use pgd_index(PAGE_OFFSET) here as index to PGD?
>
> > +             pfn_pgd(PFN_DOWN(map_pa), tableprot);
>
> It seems that __load_pa result is always used with PFN_DOWN(), it's worth
> adding __load_pfn(). Then the last two statements become
>
>         map_pfn = __load_pfn(trampoline_pmd, load_pa);
>         pgdp[pgd_index(PAGE_OFFSET)] = pfn_pgd(map_pfn, tableprot);
>
> This applies to most of the mappings below as well.

Thanks for the comments.

I am going to drop this patch because we have other patch which uses
"CFLAGS_init.o := -cmodel=medany" in mm/Makefile

Regards,
Anup