Re: [PATCH 1/2] x86: KVM: Limit guest physical bits when 5-level EPT is unsupported

[Tao Su <tao1.su@linux.intel.com>]

On Mon, Dec 18, 2023 at 07:13:27AM -0800, Sean Christopherson wrote:
> On Mon, Dec 18, 2023, Tao Su wrote:
> > When host doesn't support 5-level EPT, bits 51:48 of the guest physical
> > address must all be zero, otherwise an EPT violation always occurs and
> > current handler can't resolve this if the gpa is in RAM region. Hence,
> > instruction will keep being executed repeatedly, which causes infinite
> > EPT violation.
> > 
> > Six KVM selftests are timeout due to this issue:
> >     kvm:access_tracking_perf_test
> >     kvm:demand_paging_test
> >     kvm:dirty_log_test
> >     kvm:dirty_log_perf_test
> >     kvm:kvm_page_table_test
> >     kvm:memslot_modification_stress_test
> > 
> > The above selftests add a RAM region close to max_gfn, if host has 52
> > physical bits but doesn't support 5-level EPT, these will trigger infinite
> > EPT violation when access the RAM region.
> > 
> > Since current Intel CPUID doesn't report max guest physical bits like AMD,
> > introduce kvm_mmu_tdp_maxphyaddr() to limit guest physical bits when tdp is
> > enabled and report the max guest physical bits which is smaller than host.
> > 
> > When guest physical bits is smaller than host, some GPA are illegal from
> > guest's perspective, but are still legal from hardware's perspective,
> > which should be trapped to inject #PF. Current KVM already has a parameter
> > allow_smaller_maxphyaddr to support the case when guest.MAXPHYADDR <
> > host.MAXPHYADDR, which is disabled by default when EPT is enabled, user
> > can enable it when loading kvm-intel module. When allow_smaller_maxphyaddr
> > is enabled and guest accesses an illegal address from guest's perspective,
> > KVM will utilize EPT violation and emulate the instruction to inject #PF
> > and determine #PF error code.
> 
> No, fix the selftests, it's not KVM's responsibility to advertise the correct
> guest.MAXPHYADDR.

This patch is not for fixing these selftests, it is for fixing the issue
exposed by the selftests. KVM has responsibility to report a correct
guest.MAXPHYADDR, which lets userspace set a valid physical bits to KVM.

Actually, KVM will stuck in a loop if it tries to build spte with any bit
of bits[51:48] set, e.g., assign a huge RAM to guest.

Thanks,
Tao