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a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=BqU2WV_vvsyTyxaotp0D:22 a=pBOR-ozoAAAA:8 a=yPCof4ZbAAAA:8 a=MRADIi9n7Kqya49Bx_MA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: hQCFUNS-_cTZmqEZtWRZNxX90Xu0vBSC X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTEyMDAwMSBTYWx0ZWRfXwZBfRHc/dKFi 4PDan7zniKXIGpj0ulaESs13R9cHuu3zVmQ5+Oziq9d6UIPrekCUHxGPis7faUmTPus4qWQgYdP 3cuBjsgNsfsTPLFAx/l4fy5EuLiJ4rBTgtpJerBjfZ3wIZm0oJ/Ep0hhJnNkmxj+mCQtPqPc8zK vw6Q5hCr/wy6XGVfz9wM4MPbRLXS7j5LfwaGYUZViU8JeVKc+LhjSmn1qGC/KX4uT/NRS7qYMax obiv2/Pnylt8Zhm2QOaUV/QUzBBD2n0bJc428v61U48xbsdLcX4MxUMT/BA42aGdVV/nzD928Ll S1COP1ULdZg0o9FvkkyDWWGUK+2+Wyyf8wsEUtRlyMjUqW5sI6sqaqJ9GnGg3oNlb/8JkzvAnWi Lkoq2KwYYyK0niZMtxc+whl7zev5ViZScntGy8xaxUiwgsgC35xKOL2zu6NF8fWgoZajmFyp/xq uvIEeIB5s5IvyewInxQ== On 5/9/26 1:04 PM, David Woodhouse wrote: > On Thu, 2026-01-15 at 12:22 -0800, Dongli Zhang wrote: >> The KVM_SET_CLOCK command calls pvclock_update_vm_gtod_copy() to update the >> masterclock data. >> >> Many vCPUs may already have KVM_REQ_MASTERCLOCK_UPDATE pending before >> KVM_SET_CLOCK is invoked. If pvclock_update_vm_gtod_copy() decides to use >> the masterclock, there is no need to update the masterclock multiple times >> afterward. As noted in commit c52ffadc65e2 ("KVM: x86: Don't unnecessarily >> force masterclock update on vCPU hotplug"), each unnecessary >> KVM_REQ_MASTERCLOCK_UPDATE can cause the kvm-clock time to jump. >> >> Therefore, clear KVM_REQ_MASTERCLOCK_UPDATE for each vCPU at the end of >> KVM_SET_CLOCK when the master clock is active. The 'tsc_write_lock' ensures >> that only requests issued before KVM_SET_CLOCK are cleared. > > Hm, I think we should do this in kvm_make_mclock_inprogress_request() > instead. > > Currently, things like pvclock_gtod_update_fn() and one or two others > set KVM_REQ_MASTERCLOCK_UPDATE for all vCPUs. Not just one, because we > don't want *any* vCPU going back into guest mode before doing the work. > > So they could *all* end up calling into kvm_masterclock_update() at the > same time. I have a vague recollection there was once a mutex there, > but now there isn't. > > So all vCPUs can each, in parallel, set KVM_REQ_MCLOCK_INPROGRESS on > all other vCPUs. That's an unhandled request which just makes a vCPU > spin (preventing it from entering the guest until the clock update > completes). > > Then each vCPU tries to take the kvm->arch.tsc_write_lock in > kvm_start_pvclock_update(), and from this point they'll be > serialized... but every vCPU will go through the whole of > pvclock_update_vm_gtod_copy() for itself, updating the masterclock one > more time for each vCPU in the system? > > I came here to say that kvm_make_mclock_inprogress_request() should > probably clear KVM_REQ_MASTERCLOCK_UPDATE on all other vCPUs, since > it's about to do the work, and the other vCPUs will no longer need to. > > But... it's more broken than that now. > > I think maybe vcpu_enter_guest() should call kvm_update_masterclock() > if *kvm_test_request(KVM_REQ_MASTERCLOCK_UPDATE)* (i.e. not *clear* the > bit by using kvm_check_request()). > > And then after getting ->arch.tsc_write_lock, > kvm_start_pvclock_update() should check if KVM_REQ_MASTERCLOCK_UPDATE > is *still* set, and only proceed with the update if it is? > > I'll play with it some more... something like this perhaps? > > From 91e37d74ee267c722bc2c8f26754fcdfbc040e29 Mon Sep 17 00:00:00 2001 > From: David Woodhouse > Date: Sat, 9 May 2026 16:54:01 +0100 > Subject: [PATCH] KVM: x86: Avoid redundant masterclock updates from multiple > vCPUs > > When a masterclock update is triggered (e.g. by the clocksource change > notifier), KVM_REQ_MASTERCLOCK_UPDATE is set on all vCPUs. Without this > fix, each vCPU independently processes the request and redundantly > re-executes the entire pvclock_update_vm_gtod_copy() sequence, serialized > only by tsc_write_lock. Each redundant re-snapshot of the master clock > reference point introduces potential clock drift. > > Fix this by having __kvm_start_pvclock_update() check, after acquiring > the lock, whether the requesting vCPU's KVM_REQ_MASTERCLOCK_UPDATE is > still set. If another vCPU already did the update and cleared it, bail > out. Otherwise, clear the request on all other vCPUs before proceeding. > > The caller in vcpu_enter_guest() now uses kvm_test_request() (non-clearing) > since the clearing is done inside __kvm_start_pvclock_update() under the > lock. > > Suggested-by: Dongli Zhang > Signed-off-by: David Woodhouse Thank you very much! Feel free to add me as Suggested-by or Reported-by. I will also help test your large patch series. Regarding the issue, I would like to confirm whether there can still be any KVM_REQ_MASTERCLOCK_UPDATE after KVM_SET_CLOCK_GUEST. Per my understanding, I would expect KVM_SET_CLOCK_GUEST to be the last point where the masterclock can be updated (during live migration or live update). After KVM_SET_CLOCK_GUEST, any further masterclock change may cause kvm-clock drift. Dongli Zhang