Messages in this thread | | | Date | Fri, 8 Jul 2022 11:29:49 +0800 | Subject | Re: [PATCH v6 6/8] KVM: Handle page fault for private memory | From | 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@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.
| |