Re: [Xen-devel] [PATCH] xen/mm: Unmap page for direct mapped domain on decrease reservation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.10.15 at 12:33, <julien.grall@xxxxxxxxxx> wrote:
> Direct mapped domain needs to retrieve the exact same underlying
> physical page when the region is re-populated.
> 
> Currently, when the memory reservation for this domain is decreased, the
> request is just ignored and the page stayed mapped in the P2M. However,
> this make more difficult to spot issue when the domain has not yet mapped
> foreign page but trying to access the region.
> 
> What we really care for direct mapped domain is to not give back the
> page to the allocator. So we can re-enable to direct mapped when the guest
> memory region is re-populated.
> 
> The rest of the process to remove a page can be safely done. This
> also ensures us to stay close to the normal domain memory handling.
> 
> At the same time, drop the trailing whitespaces around the code
> modified.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

Looks good, just one minor comment (which could be addressed upon
commit):

> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -257,8 +257,16 @@ int guest_remove_page(struct domain *d, unsigned long 
> gmfn)
>  
>      if ( test_and_clear_bit(_PGT_pinned, &page->u.inuse.type_info) )
>          put_page_and_type(page);
> -            
> -    if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
> +
> +    /*
> +     * With the lack for iommu on some ARM platform, domain with DMA-capable

I think it's "lack of", and I also wonder whether calling out ARM here
is really meaningful.

> +     * device must retrieve the same pfn when the hypercall populate_physmap
> +     * is called.
> +     *
> +     * For this purpose, the page is kept allocated.

..., to match populate_physmap() behavior. (or some such)

Jan


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