[RFC PATCH 2/2] KVM: VMX: Stop/resume host PT before/after VM entry when PT_MODE_HOST_GUEST

Xiaoyao Li
2022-08-25T08:56:53Z

Current implementation in pt_guest_enter() has two issues when pt mode
is PT_MODE_HOST_GUEST.

1. It relies on VM_ENTRY_LOAD_IA32_RTIT_CTL to disable host's Intel PT
for the case that host's RTIT_CTL_TRACEEN is 1 while guest's is 0.

However, it causes VM entry failure due to violating the requirement
stated in SDM "VM-Execution Control Fields"

If the logical processor is operating with Intel PT enabled (if
IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
IA32_RTIT_CTL" VM-entry control must be 0.

2. In the case both host and guest enable Intel PT, it disables host's
Intel PT by manually clearing MSR_IA32_RTIT_CTL for the purpose to
context switch host and guest's PT configurations.

However, PT PMI can be delivered later and before VM entry. In the PT
PMI handler, it will a) update the host PT MSRs which leads to what KVM
stores in vmx->pt_desc.host becomes stale, and b) re-enable Intel PT
which leads to VM entry failure as #1.

To fix the above two issues, call intel_pt_stop() exposed by Intel PT
driver to disable Intel PT of host unconditionally, it can ensure
MSR_IA32_RTIT_CTL.TraceEn is 0 and following PT PMI does nothing.

As paired, call intel_pt_resume() after VM exit.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
arch/x86/kvm/vmx/vmx.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d7f8331d6f7e..3e9ce8f600d2 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -38,6 +38,7 @@
#include <asm/fpu/api.h>
#include <asm/fpu/xstate.h>
#include <asm/idtentry.h>
+#include <asm/intel_pt.h>
#include <asm/io.h>
#include <asm/irq_remapping.h>
#include <asm/kexec.h>
@@ -1128,13 +1129,19 @@ static void pt_guest_enter(struct vcpu_vmx *vmx)
if (vmx_pt_mode_is_system())
return;

+ /*
+ * Stop Intel PT on host to avoid vm-entry failure since
+ * VM_ENTRY_LOAD_IA32_RTIT_CTL is set
+ */
+ intel_pt_stop();
+
/*
* GUEST_IA32_RTIT_CTL is already set in the VMCS.
* Save host state before VM entry.
*/
rdmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
- wrmsrl(MSR_IA32_RTIT_CTL, 0);
+ /* intel_pt_stop() ensures RTIT_CTL.TraceEn is zero */
pt_save_msr(&vmx->pt_desc.host, vmx->pt_desc.num_address_ranges);
pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
}
@@ -1156,6 +1163,8 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
*/
if (vmx->pt_desc.host.ctl)
wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
+
+ intel_pt_resume();
}

void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
--
2.27.0