Re: [Xen-devel] [PATCH] Xen: ARM: Zero reserved fields of xatp before making hypervisor call

[Jiandi An <anjiandi@xxxxxxxxxxxxxx>]

On 12/19/16 12:49, Stefano Stabellini wrote:
> On Mon, 19 Dec 2016, Juergen Gross wrote:
>> On 19/12/16 03:56, Jiandi An wrote:
>>> Ensure all reserved fields of xatp are zero before making hypervisor
>>> call to XEN in xen_map_device_mmio().  xenmem_add_to_physmap_one() in
>>> XEN fails the mapping request if extra.res reserved field in xatp is
>>> not zero for XENMAPSPACE_dev_mmio request.
>>>
>>> Signed-off-by: Jiandi An <anjiandi@xxxxxxxxxxxxxx>
>>> ---
>>>  drivers/xen/arm-device.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/xen/arm-device.c b/drivers/xen/arm-device.c
>>> index 778acf8..208273b 100644
>>> --- a/drivers/xen/arm-device.c
>>> +++ b/drivers/xen/arm-device.c
>>> @@ -87,6 +87,9 @@ static int xen_map_device_mmio(const struct resource 
>>> *resources,
>>>                     idxs[j] = XEN_PFN_DOWN(r->start) + j;
>>>             }
>>>  
>>> +           /* Ensure reserved fields are set to zero */
>>> +           memset(&xatp, 0, sizeof(xatp));
>>> +
>>>             xatp.domid = DOMID_SELF;
>>>             xatp.size = nr;
>>>             xatp.space = XENMAPSPACE_dev_mmio;
>>
>> Instead of setting xatp to 0 in each iteration I'd prefer a static
>> initialization of .domid and .space:
>>
>>      struct xen_add_to_physmap_range xatp = {
>>              .domid = DOMID_SELF,
>>              .space = XENMAPSPACE_dev_mmio
>>      };
> 
> +1
> 

Hi Juergen,

Thanks for you comments.  xatp is passed to XEN via the hypervisor call in each 
loop.
XEN touches xatp and returns it back.  For example XEN returns error of 
underlying mapping call in the err[] array in xatp. (The err[] is not checked 
after the hypervisor call returns and it's a bug to be addressed in a separate 
patch)  XEN could theoretically corrupt xatp when it's returned.  And the loop 
would go on to the next iteration passing in whatever that's in xatp returned 
by the previous hypervisor call.  Harder to debug in my opinion if xatp get 
corrupted by XEN somehow when a bug is introduced in XEN.  At first I put the 
memset of xatp at the beginning outside of the loop.  But I thought it's better 
to initialize xatp that's passed in each time a hypervisor call is made so we 
know exactly we set going into the hypervisor call.

-- 
Jiandi An
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux 
Foundation Collaborative Project.

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