linux-kernel - [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is supported but not enabled"

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Message-Id: <20210625001853.318148-1-seanjc@google.com>
Date:   Thu, 24 Jun 2021 17:18:53 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled"

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@...gle.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);
-- 
2.32.0.93.g670b81a890-goog