Re: [Xen-devel] [PATCH v12 1/3] xen/mem_sharing: VM forking

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2020 17:54, Roger Pau Monné wrote:
> On Wed, Mar 25, 2020 at 04:42:07PM +0000, Julien Grall wrote:
>> On 25/03/2020 16:34, Tamas K Lengyel wrote:
>>>>> --- a/xen/common/domain.c
>>>>> +++ b/xen/common/domain.c
>>>>> @@ -1270,6 +1270,9 @@ int map_vcpu_info(struct vcpu *v, unsigned long 
>>>>> gfn, unsigned offset)
>>>>>
>>>>>       v->vcpu_info = new_info;
>>>>>       v->vcpu_info_mfn = page_to_mfn(page);
>>>>> +#ifdef CONFIG_MEM_SHARING
>>>>> +    v->vcpu_info_offset = offset;
>>>>
>>>> There's no need to introduce this field, you can just use v->vcpu_info
>>>> & ~PAGE_MASK AFAICT.
>>>
>>> Just doing what you suggest above results in:
>>>
>>> mem_sharing.c:1603:55: error: invalid operands to binary & (have
>>> ‘vcpu_info_t * const’ {aka ‘union <anonymous> * const’} and ‘long
>>> int’)
>>>                                       d_vcpu->vcpu_info & ~PAGE_MASK);
>>>
>>> I can of course cast the vcpu_info pointer to (long int), it's just a
>>> bit ugly. Thoughts?
>>
>> FWIW, I will also need the offset for liveupdate. I have used (unsigned
>> long)v->vcpu_info & ~PAGE_MASK so far but this is not really pretty.
> 
> I think using:
> 
> (vaddr_t)v->vcpu_info & ~PAGE_MASK
> 
> is acceptable, but that would require adding a vaddr_t typedef to x86.

I don't think vaddr_t is necessary given that all over the place we
assume conversions between pointers and unsigned long to be fine.

Jan