On Mon, Jan 08, 2024, Tao Su wrote: > On Wed, Dec 20, 2023 at 05:42:56AM -0800, Jim Mattson wrote: > > On Mon, Dec 18, 2023 at 6:08 AM Tao Su <tao1.su@xxxxxxxxxxxxxxx> wrote: > > > > > > With 4-level EPT, bits 51:48 of the guest physical address must all > > > be zero; otherwise, an EPT violation always occurs, which is an unexpected > > > VM exit in KVM currently. > > > > > > Even though KVM advertises the max physical bits to guest, guest may > > > ignore MAXPHYADDR in CPUID and set a bigger physical bits to KVM. > > > Rejecting invalid guest physical bits on KVM side is a choice, but it will > > > break current KVM ABI, e.g., current QEMU ignores the physical bits > > > advertised by KVM and uses host physical bits as guest physical bits by > > > default when using '-cpu host', although we would like to send a patch to > > > QEMU, it will still cause backward compatibility issues. > > > > > > For GPA that can't be translated by EPT but within host.MAXPHYADDR, > > > emulation should be the best choice since KVM will inject #PF for the > > > invalid GPA in guest's perspective and try to emulate the instructions > > > which minimizes the impact on guests as much as possible. > > > > > > Signed-off-by: Tao Su <tao1.su@xxxxxxxxxxxxxxx> > > > Tested-by: Yi Lai <yi1.lai@xxxxxxxxx> > > > --- > > > arch/x86/kvm/vmx/vmx.c | 7 +++++++ > > > 1 file changed, 7 insertions(+) > > > > > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > > > index be20a60047b1..a8aa2cfa2f5d 100644 > > > --- a/arch/x86/kvm/vmx/vmx.c > > > +++ b/arch/x86/kvm/vmx/vmx.c > > > @@ -5774,6 +5774,13 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu) > > > > > > vcpu->arch.exit_qualification = exit_qualification; > > > > > > + /* > > > + * Emulate the instruction when accessing a GPA which is set any bits > > > + * beyond guest-physical bits that EPT can translate. > > > + */ > > > + if (unlikely(gpa & rsvd_bits(kvm_mmu_tdp_maxphyaddr(), 63))) > > > + return kvm_emulate_instruction(vcpu, 0); > > > + > > > > This doesn't really work, since the KVM instruction emulator is > > woefully incomplete. To make this work, first you have to teach the > > KVM instruction emulator how to emulate *all* memory-accessing > > instructions. > > Please forget allow_smaller_maxphyaddr and #PF for a while. > > I agree KVM instruction emulator is incomplete. However, hardware can’t > execute instructions with GPA>48-bit and exits to KVM, KVM just repeatedly > builds SPTE, I.e., current KVM is buggy. Eh, hardware is just as much to blame as KVM. Garbage in, garbage out. > In this case, emulation may be a choice, Yes, it can be a choice, but that choice needs to be made conciously by userspace, not silently by KVM.
