linux-kernel - Re: [RFC PATCH 16/16] xen/grant-table: host_addr fixup in mapping on xenhost

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Date:   Tue, 18 Jun 2019 20:02:01 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     pbonzini@...hat.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, sstabellini@...nel.org,
        joao.m.martins@...cle.com
Subject: Re: [RFC PATCH 16/16] xen/grant-table: host_addr fixup in mapping on
 xenhost_r0

On 6/17/19 3:55 AM, Juergen Gross wrote:
> 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@...cle.com>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> 
> Without seeing the __xenbus_map_ring() modification it is impossible to
> do a proper review of this patch.
Will do in v2.

Ankur

> 
> 
> Juergen