[PATCH] KVM: PPC: Book3S HV: Provide mode where all vCPUs on a core must be the same VM

[PATCH] KVM: PPC: Book3S HV: Provide mode where all vCPUs on a core must be the same VM

[Paul Mackerras]

This adds a mode where the vcore scheduling logic in HV KVM limits itself
to scheduling only virtual cores from the same VM on any given physical
core.  This is enabled via a new module parameter on the kvm-hv module
called "one_vm_per_core".  For this to work on POWER9, it is necessary to
set indep_threads_mode=N.  (On POWER8, hardware limitations mean that KVM
is never in independent threads mode, regardless of the indep_threads_mode
setting.)

Thus the settings needed for this to work are:

1. The host is in SMT1 mode.
2. On POWER8, the host is not in 2-way or 4-way static split-core mode.
3. On POWER9, the indep_threads_mode parameter is N.
4. The one_vm_per_core parameter is Y.

With these settings, KVM can run up to 4 vcpus on a core at the same
time on POWER9, or up to 8 vcpus on POWER8 (depending on the guest
threading mode), and will ensure that all of the vcpus belong to the
same VM.

This is intended for use in security-conscious settings where users are
concerned about possible side-channel attacks between threads which could
perhaps enable one VM to attack another VM on the same core, or the host.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
---
 arch/powerpc/kvm/book3s_hv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 3e3a715..4fd04b3 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -104,6 +104,10 @@ static bool indep_threads_mode = true;
 module_param(indep_threads_mode, bool, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(indep_threads_mode, "Independent-threads mode (only on POWER9)");
 
+static bool one_vm_per_core;
+module_param(one_vm_per_core, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(one_vm_per_core, "Only run vCPUs from the same VM on a core (requires indep_threads_mode=N)");
+
 #ifdef CONFIG_KVM_XICS
 static struct kernel_param_ops module_param_ops = {
 	.set = param_set_int,
@@ -2493,6 +2497,10 @@ static bool can_dynamic_split(struct kvmppc_vcore *vc, struct core_info *cip)
 	if (!cpu_has_feature(CPU_FTR_ARCH_207S))
 		return false;
 
+	/* In one_vm_per_core mode, require all vcores to be from the same vm */
+	if (one_vm_per_core && vc->kvm != cip->vc[0]->kvm)
+		return false;
+
 	/* Some POWER9 chips require all threads to be in the same MMU mode */
 	if (no_mixing_hpt_and_radix &&
 	    kvm_is_radix(vc->kvm) != kvm_is_radix(cip->vc[0]->kvm))
-- 
2.7.4