x86/xen: update xen_oldmem_pfn_is_ram() documentation

After removing /dev/kmem, sanitizing /proc/kcore and handling /dev/mem,
this series tackles the last sane way how a VM could accidentially access
logically unplugged memory managed by a virtio-mem device: /proc/vmcore

When dumping memory via "makedumpfile", PG_offline pages, used by
virtio-mem to flag logically unplugged memory, are already properly
excluded; however, especially when accessing/copying /proc/vmcore "the
usual way", we can still end up reading logically unplugged memory part of
a virtio-mem device.

Patch #1-#3 are cleanups.  Patch #4 extends the existing oldmem_pfn_is_ram
mechanism.  Patch #5-torvalds#7 are virtio-mem refactorings for patch torvalds#8, which
implements the virtio-mem logic to query the state of device blocks.

Patch torvalds#8:

"
Although virtio-mem currently supports reading unplugged memory in the
hypervisor, this will change in the future, indicated to the device via
a new feature flag. We similarly sanitized /proc/kcore access recently.
[...]
Distributions that support virtio-mem+kdump have to make sure that the
virtio_mem module will be part of the kdump kernel or the kdump initrd;
dracut was recently [2] extended to include virtio-mem in the generated
initrd. As long as no special kdump kernels are used, this will
automatically make sure that virtio-mem will be around in the kdump initrd
and sanitize /proc/vmcore access -- with dracut.
"

This is the last remaining bit to support
VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE [3] in the Linux implementation of
virtio-mem.

Note: this is best-effort.  We'll never be able to control what runs
inside the second kernel, really, but we also don't have to care: we only
care about sane setups where we don't want our VM getting zapped once we
touch the wrong memory location while dumping.  While we usually expect
sane setups to use "makedumfile", nothing really speaks against just
copying /proc/vmcore, especially in environments where HWpoisioning isn't
typically expected.  Also, we really don't want to put all our trust
completely on the memmap, so sanitizing also makes sense when just using
"makedumpfile".

[1] https://lkml.kernel.org/r/20210526093041.8800-1-david@redhat.com
[2] dracutdevs/dracut#1157
[3] https://lists.oasis-open.org/archives/virtio-comment/202109/msg00021.html

This patch (of 9):

The callback is only used for the vmcore nowadays.

Link: https://lkml.kernel.org/r/20211005121430.30136-1-david@redhat.com
Link: https://lkml.kernel.org/r/20211005121430.30136-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrvsky@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

Author: davidhildenbrand

arch/x86/xen/mmu_hvm.c

@@ -9,12 +9,9 @@
 
 #ifdef CONFIG_PROC_VMCORE
 /*
- * This function is used in two contexts:
- * - the kdump kernel has to check whether a pfn of the crashed kernel
- *   was a ballooned page. vmcore is using this function to decide
- *   whether to access a pfn of the crashed kernel.
- * - the kexec kernel has to check whether a pfn was ballooned by the
- *   previous kernel. If the pfn is ballooned, handle it properly.
+ * The kdump kernel has to check whether a pfn of the crashed kernel
+ * was a ballooned page. vmcore is using this function to decide
+ * whether to access a pfn of the crashed kernel.
  * Returns 0 if the pfn is not backed by a RAM page, the caller may
  * handle the pfn special in this case.
  */