linux-kernel - Re: [PATCH] xen/privcmd: Support correctly 64KB page granularity when mapping memory

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Message-ID: <cfddbb3f-25fe-0b0d-5e8f-24d68b931dd0@oracle.com>
Date:   Thu, 1 Jun 2017 11:16:46 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Julien Grall <julien.grall@....com>, xen-devel@...ts.xen.org
Cc:     sstabellini@...nel.org, jgross@...e.com,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Feng Kan <fkan@....com>
Subject: Re: [PATCH] xen/privcmd: Support correctly 64KB page granularity when
 mapping memory

On 06/01/2017 10:01 AM, Julien Grall wrote:
> Hi Boris,
>
> On 01/06/17 14:33, Boris Ostrovsky wrote:
>> On 06/01/2017 08:50 AM, Julien Grall wrote:
>>> Hi Boris,
>>>
>>> On 31/05/17 14:54, Boris Ostrovsky wrote:
>>>> On 05/31/2017 09:03 AM, Julien Grall wrote:
>>>>> Commit 5995a68 "xen/privcmd: Add support for Linux 64KB page
>>>>> granularity" did
>>>>> not go far enough to support 64KB in mmap_batch_fn.
>>>>>
>>>>> The variable 'nr' is the number of 4KB chunk to map. However, when
>>>>> Linux
>>>>> is using 64KB page granularity the array of pages
>>>>> (vma->vm_private_data)
>>>>> contain one page per 64KB. Fix it by incrementing st->index
>>>>> correctly.
>>>>>
>>>>> Furthermore, st->va is not correctly incremented as PAGE_SIZE !=
>>>>> XEN_PAGE_SIZE.
>>>>>
>>>>> Fixes: 5995a68 ("xen/privcmd: Add support for Linux 64KB page
>>>>> granularity")
>>>>> CC: stable@...r.kernel.org
>>>>> Reported-by: Feng Kan <fkan@....com>
>>>>> Signed-off-by: Julien Grall <julien.grall@....com>
>>>>> ---
>>>>>  drivers/xen/privcmd.c | 4 ++--
>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>>>>> index 7a92a5e1d40c..feca75b07fdd 100644
>>>>> --- a/drivers/xen/privcmd.c
>>>>> +++ b/drivers/xen/privcmd.c
>>>>> @@ -362,8 +362,8 @@ static int mmap_batch_fn(void *data, int nr,
>>>>> void *state)
>>>>>                  st->global_error = 1;
>>>>>          }
>>>>>      }
>>>>> -    st->va += PAGE_SIZE * nr;
>>>>> -    st->index += nr;
>>>>> +    st->va += XEN_PAGE_SIZE * nr;
>>>>> +    st->index += nr / XEN_PFN_PER_PAGE;
>>>>>
>>>>>      return 0;
>>>>>  }
>>>>
>>>>
>>>> Are we still using PAGE_MASK for xen_remap_domain_gfn_array()?
>>>
>>> Do you mean in the xen_xlate_remap_gfn_array implementation? If so
>>> there are no use of PAGE_MASK as the code has been converted to
>>> support 64K page granularity.
>>>
>>> If you mean the x86 version of xen_remap_domain_gfn_array, then we
>>> don't really care as x86 only use 4KB page granularity.
>>
>>
>> I meant right above the change that you made. Should it also be replaced
>> with XEN_PAGE_MASK? (Sorry for being unclear.)
>
> Oh. The code in xen_remap_domain_gfn_array is relying on st->va to be
> page aligned. So I think we want to keep PAGE_MASK here.

Doe this imply then that 'nr' 4K pages is integral number of PAGE_SIZE
(i.e. (nr*XEN_PAGE_SIZE) % PAGE_SIZE == 0) and if yes --- do we test
this somewhere? I don't see it.

-boris