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

Xiaoyao Li
2022-07-08T03:30:10Z

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@google.com

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.