Re: [PATCH] KVM: X86: Raise #GP when clearing CR0_PG in 64 bit mode

[Sean Christopherson]

On Fri, Jun 06, 2025, H. Peter Anvin wrote:
> On 2021-12-09 09:55, Paolo Bonzini wrote:
> > On 12/7/21 10:52, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > 
> > > In the SDM:
> > > If the logical processor is in 64-bit mode or if CR4.PCIDE = 1, an
> > > attempt to clear CR0.PG causes a general-protection exception (#GP).
> > > Software should transition to compatibility mode and clear CR4.PCIDE
> > > before attempting to disable paging.
> > > 
> > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > ---
> > >   arch/x86/kvm/x86.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 00f5b2b82909..78c40ac3b197 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -906,7 +906,8 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned
> > > long cr0)
> > >           !load_pdptrs(vcpu, kvm_read_cr3(vcpu)))
> > >           return 1;
> > > -    if (!(cr0 & X86_CR0_PG) && kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE))
> > > +    if (!(cr0 & X86_CR0_PG) &&
> > > +        (is_64_bit_mode(vcpu) || kvm_read_cr4_bits(vcpu,
> > > X86_CR4_PCIDE)))
> > >           return 1;
> > >       static_call(kvm_x86_set_cr0)(vcpu, cr0);
> > > 
> > 
> > Queued, thanks.
> > 
> 
> Have you actually checked to see what real CPUs do in this case?

I have now, and EMR at least behaves as the SDM describes.  Why do you ask?


kvm_intel: Clearing CR0.PG faulted (vector = 13)


diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f79604bc0127..f90ad464ab7e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8637,6 +8637,23 @@ void vmx_exit(void)
        kvm_x86_vendor_exit();
 }
 
+static noinline void vmx_disable_paging(void)
+{
+       unsigned long cr0 = native_read_cr0();
+       long vector = -1;
+
+       asm volatile("1: mov %1, %%cr0\n\t"
+                    "   mov %2, %%cr0\n\t"
+                    "2:"
+                    _ASM_EXTABLE_FAULT(1b, 2b)
+                    : "+a" (vector)
+                    : "r" (cr0 & ~X86_CR0_PG), "r" (cr0)
+                    : "cc", "memory" );
+
+       pr_warn("Clearing CR0.PG %s (vector = %ld)\n",
+               vector < 0 ? "succeeded" : "faulted", vector);
+}
+
 int __init vmx_init(void)
 {
        int r, cpu;
@@ -8644,6 +8661,8 @@ int __init vmx_init(void)
        if (!kvm_is_vmx_supported())
                return -EOPNOTSUPP;
 
+       vmx_disable_paging();
+
        /*
         * Note, hv_init_evmcs() touches only VMX knobs, i.e. there's nothing
         * to unwind if a later step fails.