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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 7/24/23 09:35, Like Xu wrote:
From: Like Xu <likexu@xxxxxxxxxxx>

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@xxxxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Suggested-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
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@xxxxxxxxxxx/

  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;


I am not sure, when will kvm->arch.nr_vcpus_matched_tsc ever be nonzero again once a new generation starts? If this patch ever causes synchronizing to be false, matched will also be false when calling __kvm_synchronize_tsc.

What was wrong with Oliver's proposed patch?

Paolo




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux