[RFC: timer passthrough 5/9] KVM: vmx: use tsc_adjust to enable tsc_offset timer passthrough

Zhimin Feng
2021-02-06T00:49:22Z

when in vm:
rdtsc = host_tsc * (TSC multiplier) + tsc_offset(<0)
so when vm write tsc_deadline_msr the value always less than
tsc stampcounter msr value, the irq never be triggered.

the tsc_adjust msr use as below, host execute
rdtsc = host_tsc + tsc_adjust

when vmentry, we set the tsc_adjust equal tsc_offset and vmcs
tsc offset filed equal 0, so the vm execute rdtsc the result like this:
rdtsc = host_tsc + tsc_adjust + 0
the tsc_deadline_msr value will equal tsc stampcounter msr and
the irq will trigger success.

Signed-off-by: Zhimin Feng <fengzhimin@bytedance.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/vmx/vmx.c | 23 +++++++++++++++++++++++
2 files changed, 24 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index be8fc230f7c4..7971c9e755a4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -534,6 +534,7 @@ struct tick_device {
struct timer_passth_info {
u64 host_tscd;
bool host_in_tscdeadline;
+ u64 host_tsc_adjust;
struct clock_event_device *curr_dev;

void (*orig_event_handler)(struct clock_event_device *dev);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f824ee46e2d3..44b2fd59587e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6659,6 +6659,27 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)

bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);

+static void vmx_adjust_tsc_offset(struct kvm_vcpu *vcpu, bool to_host)
+{
+ u64 tsc_adjust;
+ struct timer_passth_info *local_timer_info;
+
+ local_timer_info = &per_cpu(passth_info, smp_processor_id());
+
+ if (to_host) {
+ tsc_adjust = local_timer_info->host_tsc_adjust;
+ wrmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust);
+ vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
+ } else {
+ rdmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust);
+ local_timer_info->host_tsc_adjust = tsc_adjust;
+
+ wrmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust + vcpu->arch.tsc_offset);
+ vmcs_write64(TSC_OFFSET, 0);
+
+ }
+}
+
static void vmx_host_lapic_timer_offload(struct kvm_vcpu *vcpu)
{
struct timer_passth_info *local_timer_info;
@@ -6690,6 +6711,7 @@ static void vmx_host_lapic_timer_offload(struct kvm_vcpu *vcpu)
PIN_BASED_VMX_PREEMPTION_TIMER);
}

+ vmx_adjust_tsc_offset(vcpu, false);
wrmsrl(MSR_IA32_TSCDEADLINE, 0);
if (vcpu->arch.tscd > guest_tscl) {
wrmsrl(MSR_IA32_TSCDEADLINE, vcpu->arch.tscd);
@@ -6711,6 +6733,7 @@ static void vmx_restore_passth_timer(struct kvm_vcpu *vcpu)
u64 guest_tscd;

if (vcpu->arch.timer_passth_enable) {
+ vmx_adjust_tsc_offset(vcpu, true);
local_timer_info = &per_cpu(passth_info, smp_processor_id());
host_tscd = local_timer_info->host_tscd;
rdmsrl(MSR_IA32_TSC_DEADLINE, guest_tscd);
--
2.11.0