Re: [Xen-devel] [PATCH] x86/mm: re-arrange get_page_from_l<N>e() vs pv_l1tf_check_l<N>e

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Aug 17, 2018 at 12:42:31AM -0600, Jan Beulich wrote:
> Restore symmetry between get_page_from_l<N>e(): pv_l1tf_check_l<N>e is
> uniformly invoked from outside of them. They're no longer getting called
> for non-present PTEs. This way the slightly odd three-way return value
> meaning of the higher level ones can also be got rid of.
> 
> Introduce local variables holding the page table entries processed, and
> use them throughout the loop bodies instead of re-reading them from the
> page table several times.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -900,8 +900,11 @@ get_page_from_l1e(
>      struct domain *real_pg_owner;
>      bool write;
>  
> -    if ( !(l1f & _PAGE_PRESENT) )
> +    if ( unlikely(!(l1f & _PAGE_PRESENT)) )
> +    {
> +        ASSERT_UNREACHABLE();
>          return 0;
> +    }

Why is this needed here? According to commit message get_page_from_l1e
shouldn't be called with non-present l1e.

Going through the code, both of the shadow path and (post-modification)
pv mm path won't call get_page_from_l1e with non-present l1e AFAICT.

Wei.

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