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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: VW74UBI61M2wd9EooPgwLq07rcQLOBHS6mvhbii0DtNwt0DNEgHXeW6VJh0lYCy3ZU3uFTCxhjMfV0yQMPbXuw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY5PR10MB6093 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-24_06,2025-09-24_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 phishscore=0 bulkscore=0 spamscore=0 mlxlogscore=999 mlxscore=0 suspectscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2508110000 definitions=main-2509240183 X-Authority-Analysis: v=2.4 cv=C5XpyRP+ c=1 sm=1 tr=0 ts=68d45a53 cx=c_pps a=XiAAW1AwiKB2Y8Wsi+sD2Q==:117 a=XiAAW1AwiKB2Y8Wsi+sD2Q==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=GoEa3M9JfhUA:10 a=bcS_L1TNRa0BnfPIa4MA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: 9e-SAI4i4cElqbMCc4jB4fazitbMBHus X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTIwMDAxNiBTYWx0ZWRfX20+SJkjOOovS beHKzw2suaPohrtLeJ3GtviUws7Ks2H3GJ+oGvipCVVPXYgiy4u5VvCu3wAHYenwu2f9jUsX7qE JMglTc0s7ExBG8O1i/SkmujRubDMRWUpTPgdwmdrSdhPyI/0Vabv7Ytd3poL11yueQuEwQLwaG3 mmdEcajHb7cKRK1OfFMlR+8BToibzqaAsTlMMZpJrYzGBevBDAqBRGYuzfVBqTdqdrbV45texGw UVhQcdR455b1qiPs28pYrUuN/P4yJQlbdGbhyV/V7kb4MsTZXaArJElaqr91y4B8wXvlkNVWGNZ hBBnrAXT+0t20/mYB6PTEMnvB+C6oSPiMRrnCkcy5qYOw7R54wkUOgs6iWt5mTe1SQ7bwOOhVzq LfPj6PNh X-Proofpoint-ORIG-GUID: 9e-SAI4i4cElqbMCc4jB4fazitbMBHus On 9/23/25 10:47 AM, David Woodhouse wrote: > On Tue, 2025-09-23 at 10:25 -0700, Dongli Zhang wrote: >> >> >> On 9/23/25 9:26 AM, David Woodhouse wrote: >>> On Mon, 2025-09-22 at 12:37 -0700, Dongli Zhang wrote: >>>> On 9/22/25 11:16 AM, David Woodhouse wrote: >> >> [snip] >> >>>>> >>>>>> >>>>>> As demonstrated in my test, currently guest_tsc doesn't stop counting during >>>>>> blackout because of the lack of "MSR_IA32_TSC put" at >>>>>> kvmclock_vm_state_change(). Per my understanding, it is a bug and we may need to >>>>>> fix it. >>>>>> >>>>>> BTW, kvmclock_vm_state_change() already utilizes KVM_SET_CLOCK to re-configure >>>>>> kvm-clock before continuing the guest VM. >>> >>> Yeah, right now it's probably just introducing errors for a stop/start >>> of the VM. >> >> But that help can meet the expectation? >> >> Thanks to KVM_GET_CLOCK and KVM_SET_CLOCK, QEMU saves the clock with >> KVM_GET_CLOCK when the VM is stopped, and restores it with KVM_SET_CLOCK when >> the VM is continued. > > It saves the actual *value* of the clock. I would prefer to phrase that > as "it makes the clock jump backwards to the time at which the guest > was paused". > >> This ensures that the clock value itself does not change between stop and cont. >> >> However, QEMU does not adjust the TSC offset via MSR_IA32_TSC during stop. >> >> As a result, when execution resumes, the guest TSC suddenly jumps forward. > > Oh wow, that seems really broken. If we're going to make it experience > a time warp, we should at least be *consistent*. > > So a guest which uses the TSC for timekeeping should be mostly > unaffected by this and its wallclock should still be accurate. A guest > which uses the KVM clock will be hosed by it. > > I think we should fix this so that the KVM clock is unaffected too. >From my understanding of your reply, the kvm-clock/tsc should always be adjusted whenever a QEMU VM is paused and then resumed (i.e. via stop/cont). This applies to: - stop / cont - savevm / loadvm - live migration - cpr It is a bug if the clock jumps backwards to the time at which the guest was paused. The time elapsed while the VM is paused should always be accounted for and reflected in kvm-clock/tsc once the VM resumes. Thank you very much! Dongli Zhang