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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Mar 25, 2020 at 11:00:05AM -0600, Tamas K Lengyel wrote:
> On Wed, Mar 25, 2020 at 10:52 AM Julien Grall <julien@xen.org> wrote:
> >
> >
> >
> > On 25/03/2020 16:47, Tamas K Lengyel wrote:
> > > On Wed, Mar 25, 2020 at 10:42 AM Julien Grall <julien@xen.org> 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?

page.h would be better, albeit you will have to duplicate it for x86
and Arm. There's xen/pfn.h which is not arch specific, but feels a bit
weird to place it there.

Thanks, Roger.