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

[Chao Gao <chao.gao@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.

In this case, host.MAXPHYADDR is 52 and EPT supports 4-level only thus can
translate up to 48 bits of GPA.

Here nothing visible to selftests or QEMU indicates that guest.MAXPHYADDR = 52
is invalid/incorrect. how can we say selftests are at fault and we should fix
them?