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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: Ge3RqzPXp9NiYXT6dQAbsouC+krOlftumxbhGLSzcuIA+0PsOPfTydcluJei+qAYA/O0evd7nGd3lhPwUa/vRhijT5PMhge7S42Ost+jNZI= X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY5PR10MB6117 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-04-23_07,2025-04-22_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 bulkscore=0 mlxscore=0 spamscore=0 malwarescore=0 mlxlogscore=999 suspectscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2504070000 definitions=main-2504230070 X-Proofpoint-GUID: lMQLagew8Ag2OrjabXD7DY_pwJzYOeec X-Proofpoint-ORIG-GUID: lMQLagew8Ag2OrjabXD7DY_pwJzYOeec X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNDIzMDA3MSBTYWx0ZWRfX04HSG3aWIFva /dTHv0gpnfX/DN57B9k1GOZi+UtCGAif9NhNU9q2d3yEBvWY2o3BrK3YqXlB1v9Pg+EdzN888IY 0XHFiTwE4Sx5KRn21Y74C7ShXgC80aCo4RQoLW1mb9V0S2irf0oYdQmAGm0yK8imFWoKAw0kIo2 VD5rI28HaVVuJQDl6ztz00Qc/4oV3V+lj/UyspmOLTcH/QgDfi1vIH92hIqQn+II4hGS6PzOq56 Yaug4R0JpzoM+oth4sk6syMYRWnLMKT1IIsBsZSMhGp6JrXUSKRueWmqoie3uwbwrQsZOAOM7Jt 05nq4SoXlcG9rvVgTloCLmBTvJAS1yMqTrhX+/LOCZLsB8h34P1fyTQtXOcFopqlwtl8krgwJu6 Q2t4Gugx On 18/04/2025 19:48, Sean Christopherson wrote: > On Fri, Apr 18, 2025, Vasant Hegde wrote: >> On 4/9/2025 5:26 PM, Joao Martins wrote: >>> On 04/04/2025 20:39, Sean Christopherson wrote: >>>> Add plumbing to the AMD IOMMU driver to allow KVM to control whether or >>>> not an IRTE is configured to generate GA log interrupts. KVM only needs a >>>> notification if the target vCPU is blocking, so the vCPU can be awakened. >>>> If a vCPU is preempted or exits to userspace, KVM clears is_run, but will >>>> set the vCPU back to running when userspace does KVM_RUN and/or the vCPU >>>> task is scheduled back in, i.e. KVM doesn't need a notification. >>>> >>>> Unconditionally pass "true" in all KVM paths to isolate the IOMMU changes >>>> from the KVM changes insofar as possible. >>>> >>>> Opportunistically swap the ordering of parameters for amd_iommu_update_ga() >>>> so that the match amd_iommu_activate_guest_mode(). >>> >>> Unfortunately I think this patch and the next one might be riding on the >>> assumption that amd_iommu_update_ga() is always cheap :( -- see below. >>> >>> I didn't spot anything else flawed in the series though, just this one. I would >>> suggest holding off on this and the next one, while progressing with the rest of >>> the series. >>> >>>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c >>>> index 2e016b98fa1b..27b03e718980 100644 >>>> --- a/drivers/iommu/amd/iommu.c >>>> +++ b/drivers/iommu/amd/iommu.c >>>> -static void __amd_iommu_update_ga(struct irte_ga *entry, int cpu) >>>> +static void __amd_iommu_update_ga(struct irte_ga *entry, int cpu, >>>> + bool ga_log_intr) >>>> { >>>> if (cpu >= 0) { >>>> entry->lo.fields_vapic.destination = >>>> @@ -3783,12 +3784,14 @@ static void __amd_iommu_update_ga(struct irte_ga *entry, int cpu) >>>> entry->hi.fields.destination = >>>> APICID_TO_IRTE_DEST_HI(cpu); >>>> entry->lo.fields_vapic.is_run = true; >>>> + entry->lo.fields_vapic.ga_log_intr = false; >>>> } else { >>>> entry->lo.fields_vapic.is_run = false; >>>> + entry->lo.fields_vapic.ga_log_intr = ga_log_intr; >>>> } >>>> } >>>> >>> >>> isRun, Destination and GATag are not cached. Quoting the update from a few years >>> back (page 93 of IOMMU spec dated Feb 2025): >>> >>> | When virtual interrupts are enabled by setting MMIO Offset 0018h[GAEn] and >>> | IRTE[GuestMode=1], IRTE[IsRun], IRTE[Destination], and if present IRTE[GATag], >>> | are not cached by the IOMMU. Modifications to these fields do not require an >>> | invalidation of the Interrupt Remapping Table. >>> >>> This is the reason we were able to get rid of the IOMMU invalidation in >>> amd_iommu_update_ga() ... which sped up vmexit/vmenter flow with iommu avic. >>> Besides the lock contention that was observed at the time, we were seeing stalls >>> in this path with enough vCPUs IIRC; CCing Alejandro to keep me honest. >>> >>> Now this change above is incorrect as is and to make it correct: you will need >>> xor with the previous content of the IRTE::ga_log_intr and then if it changes >>> then you re-add back an invalidation command via >>> iommu_flush_irt_and_complete()). The latter is what I am worried will >>> reintroduce these above problem :( >>> >>> The invalidation command (which has a completion barrier to serialize >>> invalidation execution) takes some time in h/w, and will make all your vcpus >>> content on the irq table lock (as is). Even assuming you somehow move the >>> invalidation outside the lock, you will content on the iommu lock (for the> >> command queue) or best case assuming no locks (which I am not sure it is >>> possible) you will need to wait for the command to complete until you can >>> progress forward with entering/exiting. >>> >>> Unless the GALogIntr bit is somehow also not cached too which wouldn't need the >>> invalidation command (which would be good news!). Adding Suravee/Vasant here. >> >> I have checked with HW architects. Its not cached. So we don't need invalidation >> after updating GALogIntr field in IRTE. > > Woot! Better to be lucky than good :-) Probably worth using this thread Message ID as a Link: tag while the IOMMU manual isn't yet up to date with this information. That usually takes a while until formalized.