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Subject: Re: [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled"
To:     Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210625001853.318148-1-seanjc@google.com>
 <28ec9d07-756b-f546-dad1-0af751167838@redhat.com>
 <YOiFsB9vZgMcpJZu@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <fd442781-d254-13b0-db76-90c6177c3faf@redhat.com>
Date:   Fri, 9 Jul 2021 19:32:42 +0200
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On 09/07/21 19:21, 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@google.com>
>>> ---
>>>
>>> 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
> 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?
> 

For now let's revert anyway.  Thanks!

Paolo