[PATCH v2] KVM: x86/tsc: Use calculated tsc_offset before matching the fist vcpu's tsc

[Like Xu <like.xu.linux@gmail.com>]

From: Like Xu <likexu@tencent.com>

Avoid using kvm->arch.cur_tsc_offset until tsc sync is actually needed.

When the vcpu is created for the first time, its tsc is 0, and KVM
calculates its tsc_offset according to kvm_compute_l1_tsc_offset().
The userspace will then set the first non-zero tsc expected value for the
first guest vcpu, at this time there is no need to play the tsc synchronize
mechanism, the KVM should continue to write tsc_offset based on previously
used kvm_compute_l1_tsc_offset().

If the tsc synchronization mechanism is incorrectly applied at this point,
KVM will use the rewritten offset of the kvm->arch.cur_tsc_offset (on
tsc_stable machines) to write tsc_offset, which is not in line with the
expected tsc of the user space.

Based on the current code, the vcpu's tsc_offset is not configured as
expected, resulting in significant guest service response latency, which
is observed in our production environment.

Applying the tsc synchronization logic after the vcpu's tsc_generation
and KVM's cur_tsc_generation have completed their first match and started
keeping tracked helps to fix this issue, which also does not break any
existing guest tsc test cases.

Reported-by: Yong He <alexyonghe@tencent.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Like Xu <likexu@tencent.com>
---
V1 -> V2 Changelog:
- Test the 'kvm_vcpu_has_run(vcpu)' proposal; (Sean)
- Test the kvm->arch.user_changed_tsc proposal; (Oliver)
V1: https://lore.kernel.org/kvm/20230629164838.66847-1-likexu@tencent.com/

 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6b9bea62fb8..4724dacea2df 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2721,7 +2721,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 			 * kvm_clock stable after CPU hotplug
 			 */
 			synchronizing = true;
-		} else {
+		} else if (kvm->arch.nr_vcpus_matched_tsc) {
 			u64 tsc_exp = kvm->arch.last_tsc_write +
 						nsec_to_cycles(vcpu, elapsed);
 			u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;

base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
-- 
2.41.0