Re: [PATCH] implement flush_cache_vmap and flush_cache_vunmap for RISC-V

[Palmer Dabbelt]

On Sun, 28 Mar 2021 18:55:09 PDT (-0700), liu@xxxxxxxxxx wrote:
> This patch implements flush_cache_vmap and flush_cache_vunmap for
> RISC-V, since these functions might modify PTE. Without this patch,
> SFENCE.VMA won't be added to related codes, which might introduce a bug
> in some out-of-order micro-architecture implementations.
>
> Signed-off-by: Jiuyang Liu <liu@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cacheflush.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
> index 23ff70350992..4adf25248c43 100644
> --- a/arch/riscv/include/asm/cacheflush.h
> +++ b/arch/riscv/include/asm/cacheflush.h
> @@ -8,6 +8,14 @@
>
>  #include <linux/mm.h>
>
> +/*
> + * flush_cache_vmap and flush_cache_vunmap might modify PTE, needs SFENCE.VMA.
> + * - flush_cache_vmap is invoked after map_kernel_range() has installed the page table entries.
> + * - flush_cache_vunmap is invoked before unmap_kernel_range() deletes the page table entries

These should have line breaks.

> + */
> +#define flush_cache_vmap(start, end) flush_tlb_all()

We shouldn't need cache flushes for permission upgrades: the ISA allows 
the old mappings to be visible until a fence, but the theory is that 
window will be sort for reasonable architectures so the overhead of 
flushing the entire TLB will overwhelm the extra faults.  There are a 
handful of places where we preemptively flush, but those are generally 
because we can't handle the faults correctly.

If you have some benchmark that demonstrates a performance issue on real 
hardware here then I'm happy to talk about this further, but this 
assumption is all over arch/riscv so I'd prefer to keep things 
consistent for now.

> +#define flush_cache_vunmap(start, end) flush_tlb_all()

This one does seem necessary.

> +
>  static inline void local_flush_icache_all(void)
>  {
>  	asm volatile ("fence.i" ::: "memory");