Re: [Xen-devel] [PATCH v2 22/24] xen/arm: mm: Embed permission in the flags

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 12 Sep 2017, Julien Grall wrote:
> Currently, it is not possible to specify the permission of a new
> mapping. It would be necessary to use the function modify_xen_mappings
> with a different set of flags.
> 
> Introduce a couple of new flags for the permissions (Non-eXecutable,
> Read-Only) and also provides definition that combine the memory attribute
> and permission for common combinations.
> 
> PAGE_HYPERVISOR is now an alias to PAGE_HYPERVISOR_RW (read-write,
> non-executable mappings). This does not affect the current mapping using
> PAGE_HYPERVISOR because Xen is currently forcing all the mapping to be
> non-executable by default (see mfn_to_xen_entry).
> 
> A follow-up patch will change modify_xen_mappings to use the new flags.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
> 
>     Changes in v2:
>         - Update the commit message
> ---
>  xen/include/asm-arm/page.h | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> index 4022b7dc33..814ed126ec 100644
> --- a/xen/include/asm-arm/page.h
> +++ b/xen/include/asm-arm/page.h
> @@ -66,12 +66,28 @@
>   * Layout of the flags used for updating the hypervisor page tables
>   *
>   * [0:2] Memory Attribute Index
> + * [3:4] Permission flags
>   */
>  #define PAGE_AI_MASK(x) ((x) & 0x7U)
>  
> -#define PAGE_HYPERVISOR         (MT_NORMAL)
> -#define PAGE_HYPERVISOR_NOCACHE (MT_DEVICE_nGnRE)
> -#define PAGE_HYPERVISOR_WC      (MT_NORMAL_NC)
> +#define _PAGE_XN_BIT    3
> +#define _PAGE_RO_BIT    4
> +#define _PAGE_XN    (1U << _PAGE_XN_BIT)
> +#define _PAGE_RO    (1U << _PAGE_RO_BIT)
> +#define PAGE_XN_MASK(x) (((x) >> _PAGE_XN_BIT) & 0x1U)
> +#define PAGE_RO_MASK(x) (((x) >> _PAGE_RO_BIT) & 0x1U)
> +
> +/* Device memory will always be mapped read-write non-executable. */
> +#define _PAGE_DEVICE    _PAGE_XN
> +#define _PAGE_NORMAL    MT_NORMAL

I think I understand the intent behind these two definitions, but I find
them more confusing then useful. Specifically, I find confusing that
_PAGE_DEVICE specifies permissions but not memory attributes, while
_PAGE_NORMAL specifies memory attributes but not permissions.

I would probably remove the two definitions completely and only retain
the useful comment above them. The patch looks good aside from this
nit.



> +#define PAGE_HYPERVISOR_RO      (_PAGE_NORMAL|_PAGE_RO|_PAGE_XN)
> +#define PAGE_HYPERVISOR_RX      (_PAGE_NORMAL|_PAGE_RO)
> +#define PAGE_HYPERVISOR_RW      (_PAGE_NORMAL|_PAGE_XN)
> +
> +#define PAGE_HYPERVISOR         PAGE_HYPERVISOR_RW
> +#define PAGE_HYPERVISOR_NOCACHE (_PAGE_DEVICE|MT_DEVICE_nGnRE)
> +#define PAGE_HYPERVISOR_WC      (_PAGE_DEVICE|MT_NORMAL_NC)
>  
>  /*
>   * Defines for changing the hypervisor PTE .ro and .nx bits. This is only to 
> be

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