Re: [Xen-devel] Question about xenpage_list

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Wed, Aug 28, 2019 at 9:54 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 28.08.2019 17:51, Tamas K Lengyel wrote:
> > On Wed, Aug 28, 2019 at 9:35 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>
> >> On 28.08.2019 17:28, Tamas K Lengyel wrote:
> >>> Hi all,
> >>> I'm trying to track down how a call in common/grant_table.c to
> >>> share_xen_page_with_guest will actually populate that page into the
> >>> guest's physmap. Immediately after the call the page doesn't seem to
> >>> be present in the physmap, as share_xen_page_with_guest will just add
> >>> the page to the domain's xenpage_list linked-list:
> >>>
> >>>         unsigned long mfn;
> >>>         unsigned long gfn;
> >>>
> >>>         share_xen_page_with_guest(virt_to_page(gt->shared_raw[i]), d, 
> >>> SHARE_rw);
> >>>
> >>>         mfn = virt_to_mfn(gt->shared_raw[i]);
> >>>         gfn = mfn_to_gmfn(d, mfn);
> >>>
> >>>         gdprintk(XENLOG_INFO, "Sharing %lx -> %lx with domain %u\n",
> >>> gfn, mfn, d->domain_id);
> >>>
> >>> This results in the following:
> >>>
> >>> (XEN) grant_table.c:1820:d0v0 Sharing ffffffffffffffff -> 42c71e with 
> >>> domain 1
> >>>
> >>> AFAICT the page only gets populated into the physmap once the domain
> >>> gets unpaused. If I let the domain run and then pause it I can see
> >>> that the page is in the guest's physmap (by looping through all the
> >>> entries in its EPT):
> >>>
> >>> (XEN) mem_sharing.c:1636:d0v0 0xf2000 -> 0x42c71e is a grant mapping
> >>> shared with the guest
> >>
> >> This should be the result of the domain having made a respective
> >> XENMAPSPACE_grant_table request, shouldn't it?
> >>
> >
> > Do you mean the guest itself or the toolstack?
>
> The guest itself - how would the tool stack know where to put the
> frame(s)?

I don't think that makes sense. How would a guest itself now that it
needs to map that frame? When you restore the VM from a savefile, it
is already running and no firmware is going to run in it to initialize
such GFNs.

As for the toolstack, I see calls to xc_dom_gnttab_seed from the
toolstack during domain creation (be it a new domain or one being
restored from a save file) which does issue a XENMEM_add_to_physmap
with the space being specified as XENMAPSPACE_grant_table. Looks like
it gathers the GFN via xc_core_arch_get_scratch_gpfn. So it looks like
that's how its done.

Thanks,
Tamas

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