Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

Sean Christopherson
2022-07-07T20:08:15Z

On Fri, Jul 01, 2022, Xiaoyao Li wrote:
> On 7/1/2022 6:21 AM, Michael Roth wrote:
> > On Thu, Jun 30, 2022 at 12:14:13PM -0700, Vishal Annapurve wrote:
> > > With transparent_hugepages=always setting I see issues with the
> > > current implementation.

...

> > > Looks like with transparent huge pages enabled kvm tried to handle the
> > > shared memory fault on 0x84d gfn by coalescing nearby 4K pages
> > > to form a contiguous 2MB page mapping at gfn 0x800, since level 2 was
> > > requested in kvm_mmu_spte_requested.
> > > This caused the private memory contents from regions 0x800-0x84c and
> > > 0x86e-0xa00 to get unmapped from the guest leading to guest vm
> > > shutdown.
> >
> > Interesting... seems like that wouldn't be an issue for non-UPM SEV, since
> > the private pages would still be mapped as part of that 2M mapping, and
> > it's completely up to the guest as to whether it wants to access as
> > private or shared. But for UPM it makes sense this would cause issues.
> >
> > >
> > > Does getting the mapping level as per the fault access type help
> > > address the above issue? Any such coalescing should not cross between
> > > private to
> > > shared or shared to private memory regions.
> >
> > Doesn't seem like changing the check to fault->is_private would help in
> > your particular case, since the subsequent host_pfn_mapping_level() call
> > only seems to limit the mapping level to whatever the mapping level is
> > for the HVA in the host page table.
> >
> > Seems like with UPM we need some additional handling here that also
> > checks that the entire 2M HVA range is backed by non-private memory.
> >
> > Non-UPM SNP hypervisor patches already have a similar hook added to
> > host_pfn_mapping_level() which implements such a check via RMP table, so
> > UPM might need something similar:
> >
> > https://github.com/AMDESE/linux/commit/ae4475bc740eb0b9d031a76412b0117339794139
> >
> > -Mike
> >
>
> For TDX, we try to track the page type (shared, private, mixed) of each gfn
> at given level. Only when the type is shared/private, can it be mapped at
> that level. When it's mixed, i.e., it contains both shared pages and private
> pages at given level, it has to go to next smaller level.
>
> https://github.com/intel/tdx/commit/ed97f4042eb69a210d9e972ccca6a84234028cad

Hmm, so a new slot->arch.page_attr array shouldn't be necessary, KVM can instead
update slot->arch.lpage_info on shared<->private conversions. Detecting whether
a given range is partially mapped could get nasty if KVM defers tracking to the
backing store, but if KVM itself does the tracking as was previously suggested[*],
then updating lpage_info should be relatively straightfoward, e.g. use
xa_for_each_range() to see if a given 2mb/1gb range is completely covered (fully
shared) or not covered at all (fully private).

[*] https://lore.kernel.org/all/YofeZps9YXgtP3f1@google.com