On Fri, Apr 24, 2020 at 3:44 AM Roger Pau Monné <roger....@citrix.com> wrote: > > On Thu, Apr 23, 2020 at 08:30:06AM -0700, Tamas K Lengyel wrote: > > When resetting a VM fork we ought to only remove pages that were allocated > > for > > the fork during it's execution and the contents copied over from the parent. > > This can be determined if the page is sharable as special pages used by the > > fork for other purposes will not pass this test. Unfortunately during the > > fork > > reset loop we only partially check whether that's the case. A page's type > > may > > indicate it is sharable (pass p2m_is_sharable) but that's not a sufficient > > check by itself. All checks that are normally performed before a page is > > converted to the sharable type need to be performed to avoid removing pages > > from the p2m that may be used for other purposes. For example, currently the > > reset loop also removes the vcpu info pages from the p2m, potentially > > putting > > the guest into infinite page-fault loops. > > > > Signed-off-by: Tamas K Lengyel <tamas.leng...@intel.com> > > Reviewed-by: Roger Pau Monné <roger....@citrix.com>
Thanks! > > I've been looking however and I'm not able to spot where you copy the > shared_info data, which I think it's also quite critical for the > domain, as it contains the info about event channels (when using L2). > Also for HVM forks the shared info should be mapped at the same gfn as > the parent, or else the child trying to access it will trigger an EPT > fault that won't be able to populate such gfn, because the shared_info > on the parent is already shared between Xen and the parent. Pages that cause an EPT fault but can't be made shared get a new page allocated for them and copied in mem_sharing_fork_page. There are many pages like that, mostly due to QEMU mapping them and thus holding an extra reference. That said, shared_info is manually copied during forking in copy_special_pages, at the end of the function you will find: old_mfn = _mfn(virt_to_mfn(d->shared_info)); new_mfn = _mfn(virt_to_mfn(cd->shared_info)); copy_domain_page(new_mfn, old_mfn); Cheers, Tamas