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

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Date:   Thu, 1 Jun 2017 16:38:44 +0100
From:   Julien Grall <julien.grall@....com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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

Hi Boris,

On 01/06/17 16:16, Boris Ostrovsky wrote:
> 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.

nr might be smaller for the last batch. But all the intermediate batch 
should have ((nr * XEN_PAGE_SIZE) % PAGE_SIZE == 0).

I think the BUILD_BUG_ON in privcmd_ioctl_mmap_batch ensure that all the 
intermediate batch will always be an integral number of PAGE_SIZE.

Cheers,

-- 
Julien Grall