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3b1526ddb2
Add optional support for device notifications in VMClock. When supported, the hypervisor will send a device notification every time it updates the seq_count to a new even value. Moreover, add support for poll() in VMClock as a means to propagate this notification to user space. poll() will return a POLLIN event to listeners every time seq_count changes to a value different than the one last seen (since open() or last read()/pread()). This means that when poll() returns a POLLIN event, listeners need to use read() to observe what has changed and update the reader's view of seq_count. In other words, after a poll() returned, all subsequent calls to poll() will immediately return with a POLLIN event until the listener calls read(). The device advertises support for the notification mechanism by setting flag VMCLOCK_FLAG_NOTIFICATION_PRESENT in vmclock_abi flags field. If the flag is not present the driver won't setup the ACPI notification handler and poll() will always immediately return POLLHUP. Signed-off-by: Babis Chalios <bchalios@amazon.es> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Signed-off-by: Takahiro Itazuri <itazur@amazon.com> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk> Tested-by: Takahiro Itazuri <itazur@amazon.com> Link: https://patch.msgid.link/20260130173704.12575-3-itazur@amazon.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>