Re: [Xen-devel] [PATCH 04/15] xen/arm: p2m: Reduce the locking section in get_page_from_gva

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 16 Jul 2018, Julien Grall wrote:
> The p2m lock is only necessary to prevent gvirt_to_maddr failing when
> break-before-make sequence is used in the P2M update concurrently on
> another pCPU. So reduce the locking section.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

> ---
> 
>     Note a newline has been dropped to keep the block together.
> ---
>  xen/arch/arm/p2m.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 14791388ad..5ca7ffe41b 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1415,9 +1415,13 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      if ( v != current )
>          return NULL;
>  
> +    /*
> +     * The lock is here to protect us against the break-before-make
> +     * sequence used when updating the entry.
> +     */
>      p2m_read_lock(p2m);
> -
>      par = gvirt_to_maddr(va, &maddr, flags);
> +    p2m_read_unlock(p2m);
>  
>      if ( par )
>      {
> @@ -1445,8 +1449,6 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      }
>  
>  err:
> -    p2m_read_unlock(p2m);
> -
>      if ( !page && p2m->mem_access_enabled )
>          page = p2m_mem_access_check_and_get_page(va, flags, v);
>  
> -- 
> 2.11.0
> 

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