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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, Mar 25, 2020 at 10:52 AM Julien Grall <julien@xxxxxxx> wrote:
>
>
>
> On 25/03/2020 16:47, Tamas K Lengyel wrote:
> > On Wed, Mar 25, 2020 at 10:42 AM Julien Grall <julien@xxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >> On 25/03/2020 16:34, Tamas K Lengyel wrote:
> >>>>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> >>>>> index 9f51370327..1ed7d13084 100644
> >>>>> --- a/xen/arch/x86/mm/p2m.c
> >>>>> +++ b/xen/arch/x86/mm/p2m.c
> >>>>> @@ -509,6 +509,12 @@ mfn_t __get_gfn_type_access(struct p2m_domain 
> >>>>> *p2m, unsigned long gfn_l,
> >>>>>
> >>>>>        mfn = p2m->get_entry(p2m, gfn, t, a, q, page_order, NULL);
> >>>>>
> >>>>> +    /* Check if we need to fork the page */
> >>>>> +    if ( (q & P2M_ALLOC) && p2m_is_hole(*t) &&
> >>>>> +         !mem_sharing_fork_page(p2m->domain, gfn, q & P2M_UNSHARE) )
> >>>>> +        mfn = p2m->get_entry(p2m, gfn, t, a, q, page_order, NULL);
> >>>>> +
> >>>>> +    /* Check if we need to unshare the page */
> >>>>>        if ( (q & P2M_UNSHARE) && p2m_is_shared(*t) )
> >>>>>        {
> >>>>>            ASSERT(p2m_is_hostp2m(p2m));
> >>>>> @@ -588,7 +594,8 @@ struct page_info *p2m_get_page_from_gfn(
> >>>>>                return page;
> >>>>>
> >>>>>            /* Error path: not a suitable GFN at all */
> >>>>> -        if ( !p2m_is_ram(*t) && !p2m_is_paging(*t) && !p2m_is_pod(*t) )
> >>>>> +        if ( !p2m_is_ram(*t) && !p2m_is_paging(*t) && !p2m_is_pod(*t) 
> >>>>> &&
> >>>>> +             !mem_sharing_is_fork(p2m->domain) )
> >>>>>                return NULL;
> >>>>>        }
> >>>>>
> >>>>> diff --git a/xen/common/domain.c b/xen/common/domain.c
> >>>>> index b4eb476a9c..62aed53a16 100644
> >>>>> --- 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.
> >>
> >> So I am all for either a new field or a macro hiding this uglyness.
> >
> > A macro sounds like a good way to go, no need for an extra field if we
> > can calculate it based on the currently existing one. How about
> >
> > #define VCPU_INFO_OFFSET(v) (((unsigned long)v->vcpu_info) & ~PAGE_MASK)
>
> I was more thinking a generic macro to find the offset in a page.
>
> PAGE_OFFSET(ptr) ((unsigned long)(ptr) & ~PAGE_MASK)

LGTM. Should we stuff this into xen/sched.h or asm/page.h?

Tamas