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

[Xiaoyao Li]

On 7/8/2022 4:08 AM, Sean Christopherson wrote:
> 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@xxxxxxxxxx

Yes, slot->arch.page_attr was introduced to help identify whether a page 
is completely shared/private at given level. It seems XARRAY can serve 
the same purpose, though I know nothing about it. Looking forward to 
seeing the patch of using XARRAY.

yes, update slot->arch.lpage_info is good to utilize the existing logic 
and Isaku has applied it to slot->arch.lpage_info for 2MB support patches.