Re: [Xen-devel] [PATCH] xen/arm: Tighten memory attribute requirement for memory shared

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 8 Aug 2017, Julien Grall wrote:
> Xen allows shared mapping to be Normal inner-cacheable with any inner cache
> allocation strategy and no restriction of the outer-cacheability.
> 
> However, Xen is always mapping those region Normal Inner Write-Back
> Outer Write-Back Inner-shareable. Per B2.8 "Mismatched memory
> attributes" in ARM DDI 0487B.a, if the guest is not using the exact same
> memory attributes (excluding any cache allocation hints) for the shared
> region then the region will be accessed with mismatched attributes.
> 
> This will result to potential loss of coherency, and may impact the
> performance.
> 
> Given that the ARM ARM strongly recommends to avoid using mismatched
> attributes, we should impose shared region to be Normal Inner Write-Back
> Outer Write-Back Inner-shareable.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

> ---
>  xen/include/public/arch-arm.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> index bd974fb13d..8f9d06ef7f 100644
> --- a/xen/include/public/arch-arm.h
> +++ b/xen/include/public/arch-arm.h
> @@ -61,15 +61,15 @@
>   *
>   * All memory which is shared with other entities in the system
>   * (including the hypervisor and other guests) must reside in memory
> - * which is mapped as Normal Inner-cacheable. This applies to:
> + * which is mapped as Normal Inner Write-Back Outer Write-Back 
> Inner-Shareable.
> + * This applies to:
>   *  - hypercall arguments passed via a pointer to guest memory.
>   *  - memory shared via the grant table mechanism (including PV I/O
>   *    rings etc).
>   *  - memory shared with the hypervisor (struct shared_info, struct
>   *    vcpu_info, the grant table, etc).
>   *
> - * Any Inner cache allocation strategy (Write-Back, Write-Through etc)
> - * is acceptable. There is no restriction on the Outer-cacheability.
> + * Any cache allocation hints are acceptable.
>   */
>  
>  /*

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