Re: [Xen-devel] [PATCH v4 18/21] xen/arm: p2m: Clean cache PT when the IOMMU doesn't support coherent walk

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-04-22 at 14:14 +0100, Julien Grall wrote:
> Some IOMMU doesn't suppport coherent PT walk. When the p2m is shared with
> the CPU, Xen has to make sure the PT changes have reached the memory.
> 
> Introduce new IOMMU callback that will retrieve the IOMMU feature for a
> specified domain.
> 
> On ARM, the platform can contain multiple IOMMUs. Each of them may not
> have the same set of feature. The domain parameter will be used to get the
> set of features for IOMMUs used by this domain.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> 
> ---
>     Changes in v4:
>         - Patch added
> ---
>  xen/arch/arm/p2m.c              |   24 ++++++++++++++++++------
>  xen/drivers/passthrough/iommu.c |   11 +++++++++++
>  xen/include/xen/iommu.h         |    5 +++++
>  3 files changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 21219de..996d2bd 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -274,6 +274,18 @@ enum p2m_operation {
>      CACHEFLUSH,
>  };
>  
> +static void unmap_coherent_domain_page(struct domain *d, const void *va)
> +{
> +    /* Some IOMMU doesn't support coherent PT walk. When the p2m is

"don't support"

> +     * shared with the CPU, Xen has to make sure that the PT changes have
> +     * reached the memory
> +     */
> +    if ( need_iommu(d) && !iommu_has_feature(d, IOMMU_FEAT_COHERENT_WALK) )
> +        clean_xen_dcache_va_range(va, PAGE_SIZE);

This is a fairly large hammer when you might only have touched a few
bytes, if any, in the page.

Wouldn't it be better to do this is write_pte, or just after the calls
to write_pte?

iommu_has_feature has a lot of callbacks -- worth calling once in
apply_p2m_changes?

Ian



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