Re: [PATCH 03/16] xen/arm: mm: Avoid flushing the TLBs when mapping are inserted

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 20 May 2022, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> Currently, the function xen_pt_update() will flush the TLBs even when
> the mappings are inserted. This is a bit wasteful because we don't
> allow mapping replacement. Even if we were, the flush would need to
> happen earlier because mapping replacement should use Break-Before-Make
> when updating the entry.
> 
> A single call to xen_pt_update() can perform a single action. IOW, it
> is not possible to, for instance, mix inserting and removing mappings.
> Therefore, we can use `flags` to determine what action is performed.
> 
> This change will be particularly help to limit the impact of switching
> boot time mapping to use xen_pt_update().
> 
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     Changes in v4:
>         - Switch the check to a different expression that will still
>         result to the same truth table.
> 
>     Changes in v2:
>         - New patch
> ---
>  xen/arch/arm/mm.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index c4487dd7fc46..747083d820dd 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -1119,7 +1119,13 @@ static bool xen_pt_check_entry(lpae_t entry, mfn_t 
> mfn, unsigned int level,
>          /* We should be here with a valid MFN. */
>          ASSERT(!mfn_eq(mfn, INVALID_MFN));
>  
> -        /* We don't allow replacing any valid entry. */
> +        /*
> +         * We don't allow replacing any valid entry.
> +         *
> +         * Note that the function xen_pt_update() relies on this
> +         * assumption and will skip the TLB flush. The function will need
> +         * to be updated if the check is relaxed.
> +         */
>          if ( lpae_is_valid(entry) )
>          {
>              if ( lpae_is_mapping(entry, level) )
> @@ -1434,11 +1440,16 @@ static int xen_pt_update(unsigned long virt,
>      }
>  
>      /*
> -     * Flush the TLBs even in case of failure because we may have
> +     * The TLBs flush can be safely skipped when a mapping is inserted
> +     * as we don't allow mapping replacement (see xen_pt_check_entry()).
> +     *
> +     * For all the other cases, the TLBs will be flushed unconditionally
> +     * even if the mapping has failed. This is because we may have
>       * partially modified the PT. This will prevent any unexpected
>       * behavior afterwards.
>       */
> -    flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
> +    if ( !((flags & _PAGE_PRESENT) && !mfn_eq(mfn, INVALID_MFN)) )
> +        flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
>  
>      spin_unlock(&xen_pt_lock);
>  
> -- 
> 2.32.0
>