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Subject: Re: [RFC PATCH 16/16] xen/grant-table: host_addr fixup in mapping on
 xenhost_r0
To:     Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Cc:     pbonzini@redhat.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, sstabellini@kernel.org,
        joao.m.martins@oracle.com
References: <20190509172540.12398-1-ankur.a.arora@oracle.com>
 <20190509172540.12398-17-ankur.a.arora@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Message-ID: <a35ab9a8-4874-fbc8-0148-aa07543e8672@suse.com>
Date:   Mon, 17 Jun 2019 12:55:59 +0200
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On 09.05.19 19:25, Ankur Arora wrote:
> Xenhost type xenhost_r0 does not support standard GNTTABOP_map_grant_ref
> semantics (map a gref onto a specified host_addr). That's because
> since the hypervisor is local (same address space as the caller of
> GNTTABOP_map_grant_ref), there is no external entity that could
> map an arbitrary page underneath an arbitrary address.
> 
> To handle this, the GNTTABOP_map_grant_ref hypercall on xenhost_r0
> treats the host_addr as an OUT parameter instead of IN and expects the
> gnttab_map_refs() and similar to fixup any state that caches the
> value of host_addr from before the hypercall.
> 
> Accordingly gnttab_map_refs() now adds two parameters, a fixup function
> and a pointer to cached maps to fixup:
>   int gnttab_map_refs(xenhost_t *xh, struct gnttab_map_grant_ref *map_ops,
>   		    struct gnttab_map_grant_ref *kmap_ops,
> -		    struct page **pages, unsigned int count)
> +		    struct page **pages, gnttab_map_fixup_t map_fixup_fn,
> +		    void **map_fixup[], unsigned int count)
> 
> The reason we use a fixup function and not an additional mapping op
> in the xenhost_t is because, depending on the caller, what we are fixing
> might be different: blkback, netback for instance cache host_addr in
> via a struct page *, while __xenbus_map_ring() caches a phys_addr.
> 
> This patch fixes up xen-blkback and xen-gntdev drivers.
> 
> TODO:
>    - also rewrite gnttab_batch_map() and __xenbus_map_ring().
>    - modify xen-netback, scsiback, pciback etc
> 
> Co-developed-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

Without seeing the __xenbus_map_ring() modification it is impossible to
do a proper review of this patch.


Juergen