Re: [PATCH 3/4] LoongArch: Add barrier between set_pte and memory access

[maobibo]

Huacai,

On 2024/10/12 上午10:16, Huacai Chen wrote:
> Hi, Bibo,
>
> On Thu, Oct 10, 2024 at 11:50 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> > It is possible to return a spurious fault if memory is accessed
> > right after the pte is set. For user address space, pte is set
> > in kernel space and memory is accessed in user space, there is
> > long time for synchronization, no barrier needed. However for
> > kernel address space, it is possible that memory is accessed
> > right after the pte is set.
> >
> > Here flush_cache_vmap/flush_cache_vmap_early is used for
> > synchronization.
> >
> > Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> > ---
> >   arch/loongarch/include/asm/cacheflush.h | 14 +++++++++++++-
> >   1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/loongarch/include/asm/cacheflush.h b/arch/loongarch/include/asm/cacheflush.h
> > index f8754d08a31a..53be231319ef 100644
> > --- a/arch/loongarch/include/asm/cacheflush.h
> > +++ b/arch/loongarch/include/asm/cacheflush.h
> > @@ -42,12 +42,24 @@ void local_flush_icache_range(unsigned long start, unsigned long end);
> >   #define flush_cache_dup_mm(mm)                         do { } while (0)
> >   #define flush_cache_range(vma, start, end)             do { } while (0)
> >   #define flush_cache_page(vma, vmaddr, pfn)             do { } while (0)
> > -#define flush_cache_vmap(start, end)                   do { } while (0)
> >   #define flush_cache_vunmap(start, end)                 do { } while (0)
> >   #define flush_icache_user_page(vma, page, addr, len)   do { } while (0)
> >   #define flush_dcache_mmap_lock(mapping)                        do { } while (0)
> >   #define flush_dcache_mmap_unlock(mapping)              do { } while (0)
> >
> > +/*
> > + * It is possible for a kernel virtual mapping access to return a spurious
> > + * fault if it's accessed right after the pte is set. The page fault handler
> > + * does not expect this type of fault. flush_cache_vmap is not exactly the
> > + * right place to put this, but it seems to work well enough.
> > + */
> > +static inline void flush_cache_vmap(unsigned long start, unsigned long end)
> > +{
> > +       smp_mb();
> > +}
> I don't know whether this is the best API to do this, and I think
> flush_cache_vunmap() also should be a smp_mb().
I do not know neither -:(, it seems that flush_cache_vmap() is better 
than arch_sync_kernel_mappings(), since function flush_cache_vmap() is 
used in vmalloc/kasan/percpu module, however arch_sync_kernel_mappings
is only used in vmalloc.

For flush_cache_vunmap(), it is used before pte_clear(), here is usage 
example.
void vunmap_range(unsigned long addr, unsigned long end)
{
        flush_cache_vunmap(addr, end);
        vunmap_range_noflush(addr, end);
        flush_tlb_kernel_range(addr, end);
}

So I think it is not necessary to add smp_mb() in flush_cache_vunmap().

Regards
Bibo Mao
> Huacai
>
> > +#define flush_cache_vmap flush_cache_vmap
> > +#define flush_cache_vmap_early flush_cache_vmap
> > +
> >   #define cache_op(op, addr)                                             \
> >          __asm__ __volatile__(                                           \
> >          "       cacop   %0, %1                                  \n"     \
> > --
> > 2.39.3