Re: [PATCH v1 01/13] arm64: Do not enable uaccess for flush_icache_range

[Fuad Tabba <tabba@google.com>]

Hi Mark,

> > No functional change intended.
>
> There is a performance change here, since the existing
> `__flush_cache_user_range` takes IDC and DIC into account, whereas
> `invalidate_icache_by_line` does not.

You're right. There is a performance change in this patch and a couple
of the others, which I will note in v2. However, I don't think that
this patch changes the behavior when it comes to IDC and DIC, does it?

> There's also an existing oversight where `__flush_cache_user_range`
> takes ARM64_WORKAROUND_CLEAN_CACHE into account, but
> `invalidate_icache_by_line` does not. I think that's a bug that we
> should fix first, so that we can backport something to stable.

I'd be happy to address that in v2, but let me make sure I understand
the issue properly.

Errata 819472 and friends (ARM64_WORKAROUND_CLEAN_CACHE) are related
to cache maintenance operations on data caches happening concurrently
with other accesses to the same address. The two places
invalidate_icache_by_line is used in conjunction with data caches are
__flush_icache_range and __flush_cache_user_range (which share the
same code before and after my patch series). In both cases,
invalidate_icache_by_line is called after the workaround is applied.
The third and only other user of invalidate_icache_by_line is
invalidate_icache_range, which only performs cache maintenance on the
icache.

The concern is that invalidate_icache_range might be performing a
cache maintenance operation on an address concurrently with another
processor performing a dc operation on the same address. Therefore,
invalidate_icache_range should perform DC CIVAC on the line before
invalidate_icache_by_line if ARM64_WORKAROUND_CLEAN_CACHE applies. Is
that right?

https://documentation-service.arm.com/static/5fa29fddb209f547eebd361d

> Arguably
> similar is true in `swsusp_arch_suspend_exit`, but for that we could add
> a comment and always use `DC CIVAC`.

I can do that in v2 as well.

Thanks,
/fuad

> Thanks,
> Mark.
>
> > Reported-by: Catalin Marinas <catalin.marinas@arm.com>
> > Reported-by: Will Deacon <will@kernel.org>
> > Link: https://lore.kernel.org/linux-arch/20200511110014.lb9PEahJ4hVOYrbwIb_qUHXyNy9KQzNFdb_I3YlzY6A@z/
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >  arch/arm64/include/asm/assembler.h | 13 ++++--
> >  arch/arm64/mm/cache.S              | 64 +++++++++++++++++++++---------
> >  2 files changed, 54 insertions(+), 23 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index 8418c1bd8f04..6ff7a3a3b238 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -426,16 +426,21 @@ alternative_endif
> >   * Macro to perform an instruction cache maintenance for the interval
> >   * [start, end)
> >   *
> > - *   start, end:     virtual addresses describing the region
> > - *   label:          A label to branch to on user fault.
> > - *   Corrupts:       tmp1, tmp2
> > + *   start, end:     virtual addresses describing the region
> > + *   needs_uaccess:  might access user space memory
> > + *   label:          label to branch to on user fault (if needs_uaccess)
> > + *   Corrupts:       tmp1, tmp2
> >   */
> > -     .macro invalidate_icache_by_line start, end, tmp1, tmp2, label
> > +     .macro invalidate_icache_by_line start, end, tmp1, tmp2, needs_uaccess, label
> >       icache_line_size \tmp1, \tmp2
> >       sub     \tmp2, \tmp1, #1
> >       bic     \tmp2, \start, \tmp2
> >  9997:
> > +     .if     \needs_uaccess
> >  USER(\label, ic      ivau, \tmp2)                    // invalidate I line PoU
> > +     .else
> > +     ic      ivau, \tmp2
> > +     .endif
> >       add     \tmp2, \tmp2, \tmp1
> >       cmp     \tmp2, \end
> >       b.lo    9997b
> > diff --git a/arch/arm64/mm/cache.S b/arch/arm64/mm/cache.S
> > index 2d881f34dd9d..092f73acdf9a 100644
> > --- a/arch/arm64/mm/cache.S
> > +++ b/arch/arm64/mm/cache.S
> > @@ -15,30 +15,20 @@
> >  #include <asm/asm-uaccess.h>
> >
> >  /*
> > - *   flush_icache_range(start,end)
> > + *   __flush_cache_range(start,end) [needs_uaccess]
> >   *
> >   *   Ensure that the I and D caches are coherent within specified region.
> >   *   This is typically used when code has been written to a memory region,
> >   *   and will be executed.
> >   *
> > - *   - start   - virtual start address of region
> > - *   - end     - virtual end address of region
> > + *   - start         - virtual start address of region
> > + *   - end           - virtual end address of region
> > + *   - needs_uaccess - (macro parameter) might access user space memory
> >   */
> > -SYM_FUNC_START(__flush_icache_range)
> > -     /* FALLTHROUGH */
> > -
> > -/*
> > - *   __flush_cache_user_range(start,end)
> > - *
> > - *   Ensure that the I and D caches are coherent within specified region.
> > - *   This is typically used when code has been written to a memory region,
> > - *   and will be executed.
> > - *
> > - *   - start   - virtual start address of region
> > - *   - end     - virtual end address of region
> > - */
> > -SYM_FUNC_START(__flush_cache_user_range)
> > +.macro       __flush_cache_range, needs_uaccess
> > +     .if     \needs_uaccess
> >       uaccess_ttbr0_enable x2, x3, x4
> > +     .endif
> >  alternative_if ARM64_HAS_CACHE_IDC
> >       dsb     ishst
> >       b       7f
> > @@ -47,7 +37,11 @@ alternative_else_nop_endif
> >       sub     x3, x2, #1
> >       bic     x4, x0, x3
> >  1:
> > +     .if     \needs_uaccess
> >  user_alt 9f, "dc cvau, x4",  "dc civac, x4",  ARM64_WORKAROUND_CLEAN_CACHE
> > +     .else
> > +alternative_insn "dc cvau, x4",  "dc civac, x4",  ARM64_WORKAROUND_CLEAN_CACHE
> > +     .endif
> >       add     x4, x4, x2
> >       cmp     x4, x1
> >       b.lo    1b
> > @@ -58,15 +52,47 @@ alternative_if ARM64_HAS_CACHE_DIC
> >       isb
> >       b       8f
> >  alternative_else_nop_endif
> > -     invalidate_icache_by_line x0, x1, x2, x3, 9f
> > +     invalidate_icache_by_line x0, x1, x2, x3, \needs_uaccess, 9f
> >  8:   mov     x0, #0
> >  1:
> > +     .if     \needs_uaccess
> >       uaccess_ttbr0_disable x1, x2
> > +     .endif
> >       ret
> > +
> > +     .if     \needs_uaccess
> >  9:
> >       mov     x0, #-EFAULT
> >       b       1b
> > +     .endif
> > +.endm
> > +
> > +/*
> > + *   flush_icache_range(start,end)
> > + *
> > + *   Ensure that the I and D caches are coherent within specified region.
> > + *   This is typically used when code has been written to a memory region,
> > + *   and will be executed.
> > + *
> > + *   - start   - virtual start address of region
> > + *   - end     - virtual end address of region
> > + */
> > +SYM_FUNC_START(__flush_icache_range)
> > +     __flush_cache_range needs_uaccess=0
> >  SYM_FUNC_END(__flush_icache_range)
> > +
> > +/*
> > + *   __flush_cache_user_range(start,end)
> > + *
> > + *   Ensure that the I and D caches are coherent within specified region.
> > + *   This is typically used when code has been written to a memory region,
> > + *   and will be executed.
> > + *
> > + *   - start   - virtual start address of region
> > + *   - end     - virtual end address of region
> > + */
> > +SYM_FUNC_START(__flush_cache_user_range)
> > +     __flush_cache_range needs_uaccess=1
> >  SYM_FUNC_END(__flush_cache_user_range)
> >
> >  /*
> > @@ -86,7 +112,7 @@ alternative_else_nop_endif
> >
> >       uaccess_ttbr0_enable x2, x3, x4
> >
> > -     invalidate_icache_by_line x0, x1, x2, x3, 2f
> > +     invalidate_icache_by_line x0, x1, x2, x3, 1, 2f
> >       mov     x0, xzr
> >  1:
> >       uaccess_ttbr0_disable x1, x2
> > --
> > 2.31.1.607.g51e8a6a459-goog
> >

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