[Devel] [PATCH RHEL7 COMMIT] kvm/x86: clear hyperv synic pages only when guest modifies MSRs

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.16.1.vz7.30.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.16.1.vz7.30.2
------>
commit 9f24caf30ffe8c9d4e751aa38898fb33d2dbab62
Author: Evgeny Yakovlev <eyakovlev at virtuozzo.com>
Date:   Tue Apr 18 16:56:35 2017 +0400

    kvm/x86: clear hyperv synic pages only when guest modifies MSRs
    
    Existing code resets guest synic message and event flag pages to zero
    when host (but not guest) updates their guest PAs by writing to
    corresponding MSRs.
    
    This turned out to be a problem for migration code when guest had a
    SYNIC irq to inject before suspending. After resuming qemu resets MSRs to
    saved values and KVM zeroes out migrated guest memory as a side effect.
    Following that guest sees an IRQ but doesn't see any event flags in event
    flags page, skips this IRQ and hangs indefinitely.
    
    This change fixes failing migration scenario by checking that pages are
    cleared by guest MSR write only.
    
    https://jira.sw.ru/browse/PSBM-63164
    https://jira.sw.ru/browse/PSBM-64657
    
    Signed-off-by: Evgeny Yakovlev <eyakovlev at virtuozzo.com>
    Reviewed-by: Roman Kagan <rkagan at virtuozzo.com>
---
 arch/x86/kvm/hyperv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index eae314b..c8efdce 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -219,7 +219,7 @@ static int synic_set_msr(struct kvm_vcpu_hv_synic *synic,
 		synic->version = data;
 		break;
 	case HV_X64_MSR_SIEFP:
-		if (data & HV_SYNIC_SIEFP_ENABLE)
+		if (data & HV_SYNIC_SIEFP_ENABLE && !host)
 			if (kvm_clear_guest(vcpu->kvm,
 					    data & PAGE_MASK, PAGE_SIZE)) {
 				ret = 1;
@@ -230,7 +230,7 @@ static int synic_set_msr(struct kvm_vcpu_hv_synic *synic,
 			synic_exit(synic, msr);
 		break;
 	case HV_X64_MSR_SIMP:
-		if (data & HV_SYNIC_SIMP_ENABLE)
+		if (data & HV_SYNIC_SIMP_ENABLE && !host)
 			if (kvm_clear_guest(vcpu->kvm,
 					    data & PAGE_MASK, PAGE_SIZE)) {
 				ret = 1;