Re: [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

On 10/06/2017 10:32 AM, Josh Poimboeuf wrote:

On Thu, Oct 05, 2017 at 04:35:03PM -0400, Boris Ostrovsky wrote:

quoted

quoted

 #ifdef CONFIG_PARAVIRT
+/*
+ * Paravirt alternatives are applied much earlier than normal alternatives.
+ * They are only applied when running on a hypervisor.  They replace some
+ * native instructions with calls to pv ops.
+ */
+void __init apply_pv_alternatives(void)
+{
+	setup_force_cpu_cap(X86_FEATURE_PV_OPS);

Not for Xen HVM guests.

From what I can tell, HVM guests still use pv_time_ops and
pv_mmu_ops.exit_mmap, right?

Right, I forgot about that one.

quoted

quoted

+
 void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
 				     struct paravirt_patch_site *end)
 {

diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
index 4fa90006ac68..17243fe0f5ce 100644
--- a/arch/x86/kernel/cpu/hypervisor.c
+++ b/arch/x86/kernel/cpu/hypervisor.c

@@ -71,6 +71,8 @@ void __init init_hypervisor_platform(void)
 	if (!x86_hyper)
 		return;
 
+	apply_pv_alternatives();

Not for Xen PV guests who have already done this.

I think it would be harmless, but yeah, it's probably best to only write
it once.

I also wonder whether calling apply_pv_alternatives() here before
x86_hyper->init_platform() will work since the latter may be setting
those op. In fact, that's what Xen HVM does for pv_mmu_ops.exit_mmap.

-boris