On Fri, Jul 09, 2021 at 05:21:52PM +0000, Sean Christopherson wrote: > On Thu, Jul 08, 2021, Paolo Bonzini wrote: > > On 25/06/21 02:18, Sean Christopherson wrote: > > > Let KVM load if EFER.NX=0 even if NX is supported, the analysis and > > > testing (or lack thereof) for the non-PAE host case was garbage. > > > > > > If the kernel won't be using PAE paging, .Ldefault_entry in head_32.S > > > skips over the entire EFER sequence. Hopefully that can be changed in > > > the future to allow KVM to require EFER.NX, but the motivation behind > > > KVM's requirement isn't yet merged. Reverting and revisiting the mess > > > at a later date is by far the safest approach. > > > > > > This reverts commit 8bbed95d2cb6e5de8a342d761a89b0a04faed7be. > > > > > > Fixes: 8bbed95d2cb6 ("KVM: x86: WARN and reject loading KVM if NX is supported but not enabled") > > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > > --- > > > > > > Hopefully it's not too late to just drop the original patch... > > > > > > arch/x86/kvm/x86.c | 3 --- > > > 1 file changed, 3 deletions(-) > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > index 4a597aafe637..1cc02a3685d0 100644 > > > --- a/arch/x86/kvm/x86.c > > > +++ b/arch/x86/kvm/x86.c > > > @@ -10981,9 +10981,6 @@ int kvm_arch_hardware_setup(void *opaque) > > > int r; > > > rdmsrl_safe(MSR_EFER, &host_efer); > > > - if (WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_NX) && > > > - !(host_efer & EFER_NX))) > > > - return -EIO; > > > if (boot_cpu_has(X86_FEATURE_XSAVES)) > > > rdmsrl(MSR_IA32_XSS, host_xss); > > > > > > > So do we want this or "depends on X86_64 || X86_PAE"? > > Hmm, I'm leaning towards keeping !PAE support purely for testing the !PAE<->PAE > MMU transitions for nested virtualization. It's not much coverage, and the !PAE May I ask what "!PAE<->PAE MMU transition for nested virtualization" means? Running L1 KVM with !PAE and L0 in PAE? I had thought KVM can only function with PAE set(though I did not see any check of CR4 in kvm_arch_init()). Did I miss something? > NPT horror is a much bigger testing gap (because KVM doesn't support it), but on > the other hand setting EFER.NX for !PAE kernels appears to be trivial, e.g. > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S > index 67f590425d90..bfbea25a9fe8 100644 > --- a/arch/x86/kernel/head_32.S > +++ b/arch/x86/kernel/head_32.S > @@ -214,12 +214,6 @@ SYM_FUNC_START(startup_32_smp) > andl $~1,%edx # Ignore CPUID.FPU > jz .Lenable_paging # No flags or only CPUID.FPU = no CR4 > > - movl pa(mmu_cr4_features),%eax > - movl %eax,%cr4 > - > - testb $X86_CR4_PAE, %al # check if PAE is enabled > - jz .Lenable_paging > - > /* Check if extended functions are implemented */ > movl $0x80000000, %eax > cpuid > > My only hesitation is the risk of somehow breaking ancient CPUs by falling into > the NX path. Maybe try forcing EFER.NX=1 for !PAE, and fall back to requiring > PAE if that gets NAK'd or needs to be reverted for whatever reason? > One more dumb question: are you planning to set NX for linux with !PAE? Why do we need EFER in that case? Thanks! :) B.R. Yu