Hi Michael, Boots for me on an x86_64 machine. Got a typo fix and a question for you. Tagging as reviewed and tested regardless :)
> On 02/20/2026 8:40 AM Michael Kelley <[email protected]> wrote: > > > From: Michael Kelley <[email protected]> > > When checking for VMBus channel interrutps, current code always scans the /s/interrutps/interrupts > full SynIC receive interrupt bit array to get the relid of the > interrupting channels. The array has HV_EVENT_FLAGS_COUNT (2048) bits. > But VMs rarely have more than 100 channels, and the relid is typically > a small integer that is densely assigned by the Hyper-V host. It's > wasteful to scan 2048 bits when it is highly unlikely that anything will > be found past bit 100. The waste is double with Confidential VMBus because > there are two receive interrupt arrays that must be scanned: one for the > hypervisor SynIC and one for the paravisor SynIC. > > Improve the scanning by tracking the largest relid that has been offered > by the Hyper-V host. Then when checking for VMBus channel interrupts, only > scan up to this high water mark. > > When channels are rescinded, it's not worth the complexity to recalculate > the high water mark. Hyper-V tends to reuse the rescinded relids for any > new channels that are subsequently added, and the performance benefit of > exactly tracking the high water mark would be minimal. > > Signed-off-by: Michael Kelley <[email protected]> Tested-by: Roman Kisel <[email protected]> Reviewed-by: Roman Kisel <[email protected]> > --- > drivers/hv/channel_mgmt.c | 16 ++++++++++++---- > drivers/hv/hyperv_vmbus.h | 3 ++- > drivers/hv/vmbus_drv.c | 7 +------ > 3 files changed, 15 insertions(+), 11 deletions(-) > > diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c > index 74fed2c073d4..61f7dffd0f50 100644 > --- a/drivers/hv/channel_mgmt.c > +++ b/drivers/hv/channel_mgmt.c > @@ -384,8 +384,18 @@ static void free_channel(struct vmbus_channel *channel) > > void vmbus_channel_map_relid(struct vmbus_channel *channel) > { > - if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS)) > + u32 new_relid = channel->offermsg.child_relid; > + > + if (WARN_ON(new_relid >= MAX_CHANNEL_RELIDS)) > return; > + > + /* > + * This function is always called in the tasklet for the connect CPU. > + * So updating the relid hiwater mark does not need to be atomic. > + */ > + if (new_relid > READ_ONCE(vmbus_connection.relid_hiwater)) > + WRITE_ONCE(vmbus_connection.relid_hiwater, new_relid); > + > /* > * The mapping of the channel's relid is visible from the CPUs that > * execute vmbus_chan_sched() by the time that vmbus_chan_sched() will > @@ -411,9 +421,7 @@ void vmbus_channel_map_relid(struct vmbus_channel > *channel) > * of the VMBus driver and vmbus_chan_sched() can not run before > * vmbus_bus_resume() has completed execution (cf. resume_noirq). > */ > - virt_store_mb( > - vmbus_connection.channels[channel->offermsg.child_relid], > - channel); > + virt_store_mb(vmbus_connection.channels[new_relid], channel); > } > > void vmbus_channel_unmap_relid(struct vmbus_channel *channel) > diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h > index 7bd8f8486e85..2c90c81a3b0f 100644 > --- a/drivers/hv/hyperv_vmbus.h > +++ b/drivers/hv/hyperv_vmbus.h > @@ -276,8 +276,9 @@ struct vmbus_connection { > struct list_head chn_list; > struct mutex channel_mutex; > > - /* Array of channels */ > + /* Array of channel pointers, indexed by relid */ > struct vmbus_channel **channels; > + u32 relid_hiwater; > > /* > * An offer message is handled first on the work_queue, and then > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c > index 3e7a52918ce0..a96da105b593 100644 > --- a/drivers/hv/vmbus_drv.c > +++ b/drivers/hv/vmbus_drv.c > @@ -1258,17 +1258,12 @@ static void vmbus_chan_sched(void *event_page_addr) > return; > event = (union hv_synic_event_flags *)event_page_addr + > VMBUS_MESSAGE_SINT; > > - maxbits = HV_EVENT_FLAGS_COUNT; > + maxbits = READ_ONCE(vmbus_connection.relid_hiwater) + 1; Worth checking that "maxbits <= HV_EVENT_FLAGS_COUNT" to protect from corruptions, etc. or would be too paranoidal? > recv_int_page = event->flags; > > if (unlikely(!recv_int_page)) > return; > > - /* > - * Suggested-by: Michael Kelley <[email protected]> > - * One possible optimization would be to keep track of the largest > relID that's in use, > - * and only scan up to that relID. > - */ > for_each_set_bit(relid, recv_int_page, maxbits) { > void (*callback_fn)(void *context); > struct vmbus_channel *channel; > -- > 2.25.1
