linux-kernel - Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

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Date:   Tue, 5 Sep 2017 11:03:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Oscar Salvador <osalvador@...e.de>,
        Ingo Molnar <mingo@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock

On Tue, Sep 05, 2017 at 10:52:06AM +0200, Juergen Gross wrote:
> > Hmm, that might work. Could we somehow nop that call when
> > !X86_FEATURE_HYPERVISOR?, that saves native from having to do the call
> > and would be a win for everyone.
> 
> So in fact we want a "always false" shortcut for bare metal and for any
> virtualization environment selecting bare metal behavior.
> 
> I'll have a try.

Right, so for native I think you can do this:


diff --git a/arch/x86/kernel/paravirt_patch_64.c b/arch/x86/kernel/paravirt_patch_64.c
index 11aaf1eaa0e4..4e9839001291 100644
--- a/arch/x86/kernel/paravirt_patch_64.c
+++ b/arch/x86/kernel/paravirt_patch_64.c
@@ -21,6 +21,7 @@ DEF_NATIVE(, mov64, "mov %rdi, %rax");
 #if defined(CONFIG_PARAVIRT_SPINLOCKS)
 DEF_NATIVE(pv_lock_ops, queued_spin_unlock, "movb $0, (%rdi)");
 DEF_NATIVE(pv_lock_ops, vcpu_is_preempted, "xor %rax, %rax");
+DEF_NATIVE(pv_lock_ops, virt_spin_lock, "xor %rax, %rax");
 #endif
 
 unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
@@ -77,6 +78,13 @@ unsigned native_patch(u8 type, u16 clobbers, void *ibuf,
 				goto patch_site;
 			}
 			goto patch_default;
+		case PARAVIRT_PATCH(pv_lock_ops.virt_spin_lock):
+			if (!this_cpu_has(X86_FEATURE_HYPERVISOR)) {
+				start = start_pv_lock_ops_virt_spin_lock;
+				end   = end_pv_lock_ops_virt_spin_lock;
+				goto patch_side;
+			}
+			goto patch_default;
 #endif
 
 	default: