On 02.05.23 18:06, Jason Gunthorpe wrote:
On Tue, May 02, 2023 at 05:45:40PM +0200, David Hildenbrand wrote:
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.
It missses the general architectural point why we have all these
shootdown mechanims in other places - plares are not supposed to make
these kinds of assumptions. When the userspace unplugs the memory from
KVM or unmaps it from VFIO it is not still being accessed by the
kernel.
Yes. Like having memory in a vfio iommu v1 and doing the same (mremap,
munmap, MADV_DONTNEED, ...). Which is why we disable MADV_DONTNEED
(e.g., virtio-balloon) in QEMU with vfio.
Functional bug or not, it is inconsistent with how this is designed to
work.
Sorry to say, I *really* don't see how that is supposed to work with a
page that *cannot* be faulted back in on demand.
--
Thanks,
David / dhildenb
