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

[Keir Fraser <keir.xen@gmail.com>]

On 13/07/2012 21:20, "Olaf Hering" <olaf@aepfle.de> wrote:

> 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.

So pfn A now points at shared_info, and mfn M is lost (freed back to Xen).
Xen_vcpu doesn't come into it, you'd have that mapped at yet another pfn.

> 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.

Don't really understand this. After the XENMAPSPACE_shared_info_call:
 * PFN B points at shared_info, mfn M_B it previously mapped is lost (freed
back to Xen).
 * PFN A maps nothing, reads return all-1s.

Yes, obviously you can't atomically update the mapping of shinfo from A->B,
ad update your pointer in the kernel at exactly the same time. Presumably
you do this early during boot, or late during kexec, or otherwise at a time
when other processors are not expected to touch shinfo.

> 
> 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.

That is correct.

> What do you think, is that race real?

I suppose it is. I didn't imagine it would be a troublesome one though.

 -- Keir

> Olaf