Re: regression - 2.6.36 -> 2.6.37 - kvm - 32bit SMP guests don'tboot

From: Zachary Amsden
Date: Fri Feb 25 2011 - 09:45:24 EST

On 02/25/2011 05:48 AM, Nikola Ciprich wrote:
(CC: Zachary)

Zachary, in case You haven't noticed the thread, we're trying
to find out the reason why 32bit SMP guests stopped working
in 2.6.37.
bisect shows this as the culprit:

I was not aware of the thread. Please cc me directly, or add a keyword I track - timekeeping, TSC..

e48672fa25e879f7ae21785c7efd187738139593 is first bad commit
commit e48672fa25e879f7ae21785c7efd187738139593
Author: Zachary Amsden<zamsden@xxxxxxxxxx>
Date: Thu Aug 19 22:07:23 2010 -1000

KVM: x86: Unify TSC logic

Move the TSC control logic from the vendor backends into x86.c
by adding adjust_tsc_offset to x86 ops. Now all TSC decisions
can be done in one place.

Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
Signed-off-by: Marcelo Tosatti<mtosatti@xxxxxxxxxx>

That change alone may not bisect well; without further fixes on top of it, you may end up with a hang or stall, which is likely to manifest in a vendor-specific way.

Basically there were a few differences in the platform code about how TSC was dealt with on systems which did not have stable clocks, this brought the logic into one location, but there was a slight change to the logic here.

Note very carefully, the logic on SVM is gated by a condition before this change:

if (unlikely(cpu != vcpu->cpu)) {
- u64 delta;
- if (check_tsc_unstable()) {
- /*
- * Make sure that the guest sees a monotonically
- * increasing TSC.
- */
- delta = vcpu->arch.host_tsc - native_read_tsc();
- svm->vmcb->control.tsc_offset += delta;
- if (is_nested(svm))
- svm->nested.hsave->control.tsc_offset += delta;
- }
- vcpu->cpu = cpu;
- kvm_migrate_timers(vcpu);

So this only happens with a system which reports TSC as unstable. After the change, KVM itself may report the TSC as unstable:

+ if (unlikely(vcpu->cpu != cpu)) {
+ /* Make sure TSC doesn't go backwards */
+ s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
+ native_read_tsc() - vcpu->arch.last_host_tsc;
+ if (tsc_delta < 0)
+ mark_tsc_unstable("KVM discovered backwards TSC");
+ if (check_tsc_unstable())
+ kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
+ kvm_migrate_timers(vcpu);
+ vcpu->cpu = cpu;
+ }

If the platform has very small TSC deltas across CPUs, but indicates the TSC is stable, this could result in KVM marking the TSC unstable. If that is the case, this compensation logic will kick in to avoid backwards TSCs.

Note however, that the logic is not perfect; time which passes while not running on any CPU will be erased, as the delta compensation removes not just backwards, but any elapsed time from the TSC. In extreme cases, this could result in time appearing to stand still.... with guests failing to boot.

This was addressed with a later change, which catches up the missing time:

commit c285545f813d7b0ce989fd34e42ad1fe785dc65d
Author: Zachary Amsden <zamsden@xxxxxxxxxx>
Date: Sat Sep 18 14:38:15 2010 -1000

KVM: x86: TSC catchup mode

Negate the effects of AN TYM spell while kvm thread is preempted by tracking
conversion factor to the highest TSC rate and catching the TSC up when it has
fallen behind the kernel view of time. Note that once triggered, we don't
turn off catchup mode.

A slightly more clever version of this is possible, which only does catchup
when TSC rate drops, and which specifically targets only CPUs with broken
TSC, but since these all are considered unstable_tsc(), this patch covers
all necessary cases.

Signed-off-by: Zachary Amsden <zamsden@xxxxxxxxxx>
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

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