Re: [kvm-unit-tests PATCH] x86/access: Fixed test stuck issue on new 52bit machine

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 12, 2021, Yang Weijiang wrote:
> On Mon, Jan 11, 2021 at 02:25:59PM -0800, Sean Christopherson wrote:
> > On Sun, Jan 10, 2021, Yang Weijiang wrote:
> > > When the application is tested on a machine with 52bit-physical-address, the
> > > synthesized 52bit GPA triggers EPT(4-Level) fast_page_fault infinitely.
> > 
> > That doesn't sound right, KVM should use 5-level EPT if guest maxpa > 48.
> > Hmm, unless the CPU doesn't support 5-level EPT, but I didn't think such CPUs
> > (maxpa=52 w/o 5-level EPT) existed?  Ah, but it would be possible with nested
> > VMX, and initial KVM 5-level support didn't allow nested 5-level EPT.  Any
> > chance you're running this test in a VM with 5-level EPT disabled, but maxpa=52?
> >
> Hi, Sean,
> Thanks for the reply!
> I use default settings of the unit-test + 5.2.0 QEMU + 5.10 kernel, in

The default settings are supposed to set guest.MAXPA = host.MAXPA.  At least, I
assume that's the purpose of '-cpu max'.  Maybe your copy of kvm-unit-tests'
x86/unittests.cfg is stale?

  [access]
  file = access.flat
  arch = x86_64
  extra_params = -cpu max
  timeout = 180

> this case, QEMU uses cpu->phys_bits==40, so the guest's PA=40bit and
> LA=57bit, hence 5-level EPT is not enabled. My physical machine is PA=52
> and LA=57 as can checked from cpuid:
> cpuid -1r -l 0x80000008 -s 0
> CPU:
>    0x80000008 0x00: eax=0x00003934 ...
> There're two other ways to w/a this issue: 1) change the QEMU params to
> to extra_params = -cpu host,host-phys-bits, so guest's PA=52 and LA=57,
> this will enable 5-level EPT, meanwhile, it escapes the problematic GPA
> by adding AC_*_BIT51_MASK in invalid_mask.
> 
> 2) add allow_smaller_maxphyaddr=1 to kvm-intel module.

Setting allow_smaller_maxphyaddr=1 is the correct answer.