Re: [PATCH][RFC] kvm-scheduler integration

[Ingo Molnar <mingo@elte.hu>]

* Avi Kivity <avi@qumranet.com> wrote:

> Intel VT essentially introduces a new set of registers into the 
> processor; this means we cannot preempt kvm in kernel mode lest a new 
> VM run with and old VM's registers.  In addition, kvm lazy switches 
> some host registers as well. (AMD does not introduce new registers, 
> but we still want lazy msr switching, and we want to know when we move 
> to a different cpu in order to be able to guarantee a monotonously 
> increasing tsc).
> 
> Current kvm code simply disables preemption when guest context is in 
> use. This, however, has many drawbacks:
> 
> - some kvm mmu code is O(n), causing possibly unbounded latencies and causing
>   -rt great unhappiness.
> - the mmu code wants to sleep (especially with guest paging), but can't.
> - some optimizations are not possible; for example, if we switch from one
>   VM to another, we need not restore some host registers (as they will simply
>   be overwritten with the new guest registers immediately).
> 
> This patch adds hooks to the scheduler that allow kvm to be notified 
> about scheduling decisions involving virtual machines.  When we 
> schedule out a VM, kvm is told to swap guest registers out; when we 
> schedule the VM in, we swap the registers back in.

hm, why not do what i have in -rt? See the patch below. Seems to work 
fine for me, although i might be missing something.

	Ingo

------------------------->
Subject: [patch] kvm: make vcpu_load/put preemptible
From: Ingo Molnar <mingo@elte.hu>

make vcpu_load/put preemptible.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 drivers/kvm/svm.c |   13 ++++++++++---
 drivers/kvm/vmx.c |   15 ++++++++++++---
 2 files changed, 22 insertions(+), 6 deletions(-)

Index: linux-rt-rebase.q/drivers/kvm/svm.c
===================================================================
--- linux-rt-rebase.q.orig/drivers/kvm/svm.c
+++ linux-rt-rebase.q/drivers/kvm/svm.c
@@ -610,9 +610,17 @@ static void svm_free_vcpu(struct kvm_vcp
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu)
 {
-	int cpu, i;
+	int cpu = raw_smp_processor_id(), i;
+	cpumask_t this_mask = cpumask_of_cpu(cpu);
+
+	/*
+	 * Keep the context preemptible, but do not migrate
+	 * away to another CPU. TODO: make sure this persists.
+	 * Save/restore original mask.
+	 */
+	if (unlikely(!cpus_equal(current->cpus_allowed, this_mask)))
+		set_cpus_allowed(current, cpumask_of_cpu(cpu));
 
-	cpu = get_cpu();
 	if (unlikely(cpu != vcpu->cpu)) {
 		u64 tsc_this, delta;
 
@@ -638,7 +646,6 @@ static void svm_vcpu_put(struct kvm_vcpu
 		wrmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
 
 	rdtscll(vcpu->host_tsc);
-	put_cpu();
 }
 
 static void svm_vcpu_decache(struct kvm_vcpu *vcpu)
Index: linux-rt-rebase.q/drivers/kvm/vmx.c
===================================================================
--- linux-rt-rebase.q.orig/drivers/kvm/vmx.c
+++ linux-rt-rebase.q/drivers/kvm/vmx.c
@@ -241,9 +241,16 @@ static void vmcs_set_bits(unsigned long 
 static void vmx_vcpu_load(struct kvm_vcpu *vcpu)
 {
 	u64 phys_addr = __pa(vcpu->vmcs);
-	int cpu;
+	int cpu = raw_smp_processor_id();
+	cpumask_t this_mask = cpumask_of_cpu(cpu);
 
-	cpu = get_cpu();
+	/*
+	 * Keep the context preemptible, but do not migrate
+	 * away to another CPU. TODO: make sure this persists.
+	 * Save/restore original mask.
+	 */
+	if (unlikely(!cpus_equal(current->cpus_allowed, this_mask)))
+		set_cpus_allowed(current, cpumask_of_cpu(cpu));
 
 	if (vcpu->cpu != cpu)
 		vcpu_clear(vcpu);
@@ -281,7 +288,6 @@ static void vmx_vcpu_load(struct kvm_vcp
 static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
 {
 	kvm_put_guest_fpu(vcpu);
-	put_cpu();
 }
 
 static void vmx_vcpu_decache(struct kvm_vcpu *vcpu)
@@ -1862,6 +1868,7 @@ again:
 	}
 #endif
 
+	preempt_disable();
 	asm (
 		/* Store host registers */
 		"pushf \n\t"
@@ -2002,6 +2009,8 @@ again:
 
 		reload_tss();
 	}
+	preempt_enable();
+
 	++vcpu->stat.exits;
 
 #ifdef CONFIG_X86_64
-
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