Re: [PATCH v4 5/9] x86/mem-sharing: copy GADDR based shared guest areas

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 28, 2023 at 08:57:04AM -0400, Tamas K Lengyel wrote:
> On Thu, Sep 28, 2023 at 7:08 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
> >
> > On Thu, Sep 28, 2023 at 12:11:02PM +0200, Jan Beulich wrote:
> > > On 28.09.2023 11:51, Roger Pau Monné wrote:
> > > > On Thu, Sep 28, 2023 at 09:16:20AM +0200, Jan Beulich wrote:
> > > >> --- a/xen/arch/x86/mm/mem_sharing.c
> > > >> +++ b/xen/arch/x86/mm/mem_sharing.c
> > > >> @@ -1641,6 +1641,68 @@ static void copy_vcpu_nonreg_state(struc
> > > >>      hvm_set_nonreg_state(cd_vcpu, &nrs);
> > > >>  }
> > > >>
> > > >> +static int copy_guest_area(struct guest_area *cd_area,
> > > >> +                           const struct guest_area *d_area,
> > > >> +                           struct vcpu *cd_vcpu,
> > > >> +                           const struct domain *d)
> > > >> +{
> > > >> +    mfn_t d_mfn, cd_mfn;
> > > >> +
> > > >> +    if ( !d_area->pg )
> > > >> +        return 0;
> > > >> +
> > > >> +    d_mfn = page_to_mfn(d_area->pg);
> > > >> +
> > > >> +    /* Allocate & map a page for the area if it hasn't been already. 
> > > >> */
> > > >> +    if ( !cd_area->pg )
> > > >> +    {
> > > >> +        gfn_t gfn = mfn_to_gfn(d, d_mfn);
> > > >> +        struct p2m_domain *p2m = p2m_get_hostp2m(cd_vcpu->domain);
> > > >> +        p2m_type_t p2mt;
> > > >> +        p2m_access_t p2ma;
> > > >> +        unsigned int offset;
> > > >> +        int ret;
> > > >> +
> > > >> +        cd_mfn = p2m->get_entry(p2m, gfn, &p2mt, &p2ma, 0, NULL, 
> > > >> NULL);
> > > >> +        if ( mfn_eq(cd_mfn, INVALID_MFN) )
> > > >> +        {
> > > >> +            struct page_info *pg = 
> > > >> alloc_domheap_page(cd_vcpu->domain, 0);
> > > >> +
> > > >> +            if ( !pg )
> > > >> +                return -ENOMEM;
> > > >> +
> > > >> +            cd_mfn = page_to_mfn(pg);
> > > >> +            set_gpfn_from_mfn(mfn_x(cd_mfn), gfn_x(gfn));
> > > >> +
> > > >> +            ret = p2m->set_entry(p2m, gfn, cd_mfn, PAGE_ORDER_4K, 
> > > >> p2m_ram_rw,
> > > >> +                                 p2m->default_access, -1);
> > > >> +            if ( ret )
> > > >> +                return ret;
> > > >> +        }
> > > >
> > > > I'm still unsure why map_guest_area() shouldn't be able to deal with a
> > > > forked child needing the page to be mapped.  What happens when a
> > > > forked child executes the hypercall to map such areas against not yet
> > > > populated gfns?
> > > >
> > > > Shouldn't map_guest_area() be capable of handling those calls and
> > > > populating on demand?
> > >
> > > Funny you should use this wording: P2M_ALLOC will deal with populating
> > > PoD slots, yes, but aiui forks don't fill their p2m with all PoD slots,
> > > but rather leave them empty. Yet again I need to leave it to Tamas to
> > > confirm or prove me wrong.
> >
> > If the child p2m populating is only triggered by guest accesses then a
> > lot of hypercalls are likely to not work correctly on childs.
> 
> That's not what's happening. As I said before, ALL access paths, be
> that from the guest, dom0 or Xen trigger page population. If there is
> a hole and P2M_ALLOC is set we do the following in
> p2m_get_gfn_type_access:
> 
>     /* 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);
> 
> Depending on the type of access we either populate the hole with a
> shared memory entry or a copy.

Then the code here is redundant, as the call to get_page_from_gfn(...,
P2M_UNSHARE) in map_vcpu_info() will already take care of populating
the child p2m and copy the parents page contents?

Thanks, Roger.