Re: [Xen-devel] [For Xen-4.10 RFC PATCH 3/3] Prevent redundant icache flushes in populate_physmap()

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Mar 31, 2017 at 11:24:24AM +0100, Punit Agrawal wrote:
> populate_physmap() calls alloc_heap_pages() per requested extent. As
> alloc_heap_pages() performs icache maintenance operations affecting the
> entire instruction cache, this leads to redundant cache flushes when
> allocating multiple extents in populate_physmap().
> 
> To alleviate this problem, introduce a new flag "MEMF_no_icache_flush"
> which can be used prevent alloc_heap_pages() to perform unnecessary
> icache maintenance operations. Use the flag in populate_physmap() and
> perform the required icache maintenance function at the end of the
> operation.
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
> ---
>  xen/common/memory.c        | 6 ++++++
>  xen/common/page_alloc.c    | 2 +-
>  xen/include/asm-x86/page.h | 4 ++++
>  xen/include/xen/mm.h       | 2 ++
>  4 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/common/memory.c b/xen/common/memory.c
> index ad0b33ceb6..507f363924 100644
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -162,6 +162,8 @@ static void populate_physmap(struct memop_args *a)
>      if ( unlikely(!d->creation_finished) )
>          a->memflags |= MEMF_no_tlbflush;
>  
> +    a->memflags |= MEMF_no_icache_flush;
> +

Not a good idea.

I think what you need is probably to group this under
!d->creation_finished.

>      for ( i = a->nr_done; i < a->nr_extents; i++ )
>      {
>          if ( i != a->nr_done && hypercall_preempt_check() )
> @@ -253,6 +255,10 @@ static void populate_physmap(struct memop_args *a)
>  out:
>      if ( need_tlbflush )
>          filtered_flush_tlb_mask(tlbflush_timestamp);
> +
> +    if ( a->memflags & MEMF_no_icache_flush )

This should be inverted, right?

The modification to this function is definitely wrong as-is. You end up
always setting the no flush flag and later always flushing the cache.


Wei.

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