Re: [Xen-devel] [PATCH] xen: arm: clarify cacheability requirements of hypercall arguments.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 17 Dec 2013, Ian Campbell wrote:
> Accepting hypercall arguments which are either consistently in cached or
> uncached is tricky and/or potentially slow, requiring a guest mapping lookup
> to determine whether/when to do a cache clean or invalidate.
> 
> There are very few reasons, and no current use cases in practice, for a guest
> to use uncached memory for their hypercall arguments. Therefore mandate that
> all hypercall arguments must be mapped inner-cacheable.
> 
> If use cases arise then we can consider specific exemptions to this rule.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>
>  xen/include/public/arch-arm.h |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> index 475cb4a..7b95237 100644
> --- a/xen/include/public/arch-arm.h
> +++ b/xen/include/public/arch-arm.h
> @@ -58,6 +58,9 @@
>   * (AAPCS64). Where there is a conflict the 64-bit standard should be
>   * used regardless of guest type. Structures which are passed as
>   * hypercall arguments are always little endian.
> + *
> + * All hypercall arguments passed via a pointer to guest memory must
> + * reside in memory which is mapped as Normal Inner-cacheable.

Maybe it is good to be pedantic: should we write "Inner or Outer and
Inner" cacheable?
Should we mention explicitly that "Write-Back", "Write-Through" or
"Write-Back Write-Allocate" are all supported?

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