On Tue, Jul 10, Keir Fraser wrote: > On 10/07/2012 19:09, "Olaf Hering" <olaf at aepfle.de> wrote: > > I'm not sure, most likely the gfn will just disappear from the guest, > > like a ballooned page disappears. Accessing it will likely cause a > > crash. > > Best thing to do, is possible, is map the shared-info page in the > xen-platform pci device's BAR memory range. Then it will not conflict with > any RAM. > > If you do map it over the top of an existing RAM page, you will have to > repopulate that RAM page before kexec, using populate_physmap hypercall. The > good news is that the populate_physmap hypercall will have the side effect > of unmapping the shared-info page, reayd to be mapped wherever the new > kernel would like it to reside :) Keir, is this a safe thing to do in a SMP guest? If arch/x86/xen/enlighten.c:xen_hvm_init_shared_info() allocates a page (backed by mfn M and pfn A) and assigns *HYPERVISOR_shared_info and *xen_vcpu then everything will reference these pointers. If drivers/xen/platform-pci.c:platform_pci_init would also do a XENMAPSPACE_shared_info call with pfn B, isnt there a small window where pfn A is not backed by a mfn because mfn M is now connected to pfn C? As a result other code paths which access *HYPERVISOR_shared_info and *xen_vcpu between the hypercall and the update of the pointers will read 0xff. If I read the hypercall code of XENMEM_add_to_physmap correctly the mfn backing *HYPERVISOR_shared_info will remain the same, so there is no need to copy data from the old to the new *HYPERVISOR_shared_info. What do you think, is that race real? Olaf
