On 02.05.23 17:36, Jason Gunthorpe wrote:
On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote:
How does s390 avoid mmu notifiers without having lots of problems?? It
is not really optional to hook the invalidations if you need to build
a shadow page table..
Totally no idea on s390 details, but.. per my read above, if the firmware
needs to make sure the page is always available (so no way to fault it in
on demand), which means a longterm pinning seems appropriate here.
Then if pinned a must, there's no need for mmu notifiers (as the page will
simply not be invalidated anyway)?
And what if someone deliberately changes the mapping? memory hotplug
in the VM, or whatever?
Besides s390 not supporting memory hotplug in VMs (yet): if the guest
wants a different guest physical address, I guess that's the problem of
the guest, and it can update it:
KVM_S390_ZPCIOP_REG_AEN is triggered from QEMU via
s390_pci_kvm_aif_enable(), triggered by the guest via a special instruction.
If the hypervisor changes the mapping, it's just the same thing as
mixing e.g. MADV_DONTNEED with longterm pinning in vfio: don't do it.
And if you do it, you get to keep the mess you created for your VM.
Linux will make sure to not change the mapping: for example, page
migration of a pinned page will fail.
But maybe I am missing something important here.
--
Thanks,
David / dhildenb
