Re: [Xen-devel] incorrect layout of globals from head_64.S during kexec boot

[Olaf Hering <olaf@aepfle.de>]

On Tue, Jul 10, Keir Fraser wrote:

> On 10/07/2012 19:09, "Olaf Hering" <olaf@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