From: Long Li longli@microsoft.com Sent: Wednesday, April 23, 2025 11:49 AM
Interrupt and monitor pages should be in Hyper-V page size (4k bytes). This can be different to the system page size.
I'm curious about this change. Since Patch 1 of the series changes the allocations to be the full PAGE_SIZE, what does it mean to set the mapping size to less than a full page (in the case PAGE_SIZE > HV_HYP_PAGE_SIZE)? mmap can only map full PAGE_SIZE pages, so uio_mmap() rounds up the INT_PAGE_MAP and MON_PAGE_MAP sizes to PAGE_SIZE when checking the validity of the mmap parameters.
The changes in this patch do ensure that the INT_PAGE_MAP and MON_PAGE_MAP "maps" entries in sysfs always show the size as 4096 even if the full PAGE_SIZE is actually mapped, but I'm not sure if that difference is good or bad.
Kernel needs to tell the user-mode the correct length of the map. In this case, 4096 bytes are usable data regardless of what is actual mapped as long as it's >4096.
The DPDK vmbus driver uses this length to setup checks for accessing the page.
OK, so the "maps" entry needs to show a size of 4096 so the DPDK VMBus driver does the right thing.
At a minimum, mention in the commit message that the reason for the change is to always show 4096 as the size of the "maps" entries, even if PAGE_SIZE and the actual mapped area is larger. Perhaps also mention that the DPDK VMBus driver is one known consumer of that detail.
Michael