From: Dexuan Cui <decui@xxxxxxxxxxxxx> Sent: Monday, September 2, 2019 5:23 PM > > The API will be used by the hv_balloon and hv_vmbus drivers. > > Balloon up/down and hot-add of memory must not be active if the user > wants the Linux VM to support hibernation, because they are incompatible > with hibernation according to Hyper-V team, e.g. upon suspend the > balloon VSP doesn't save any info about the ballooned-out pages (if any); > so, after Linux resumes, Linux balloon VSC expects that the VSP will > return the pages if Linux is under memory pressure, but the VSP will > never do that, since the VSP thinks it never stole the pages from the VM. > > So, if the user wants Linux VM to support hibernation, Linux must forbid > balloon up/down and hot-add, and the only functionality of the balloon VSC > driver is reporting the VM's memory pressure to the host. > > Ideally, when Linux detects that the user wants it to support hibernation, > the balloon VSC should tell the VSP that it does not support ballooning > and hot-add. However, the current version of the VSP requires the VSC > should support these capabilities, otherwise the capability negotiation > fails and the VSC can not load at all, so with the later changes to the > VSC driver, Linux VM still reports to the VSP that the VSC supports these > capabilities, but the VSC ignores the VSP's requests of balloon up/down > and hot add, and reports an error to the VSP, when applicable. BTW, in > the future the balloon VSP driver will allow the VSC to not support the > capabilities of balloon up/down and hot add. > > The ACPI S4 state is not a must for hibernation to work, because Linux is > able to hibernate as long as the system can shut down. However in practice > we decide to artificially use the presence of the virtual ACPI S4 state as > an indicator of the user's intent of using hibernation, because Linux VM > must find a way to know if the user wants to use the hibernation feature > or not. > > By default, Hyper-V does not enable the virtual ACPI S4 state; on recent > Hyper-V hosts (e.g. RS5, 19H1), the administrator is able to enable the > state for a VM by WMI commands. > > Once all the vmbus and VSC patches for the hibernation feature are > accepted, an extra patch will be submitted to forbid hibernation if the > virtual ACPI S4 state is absent, i.e. hv_is_hibernation_supported() is > false. > > Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx> > --- > arch/x86/hyperv/hv_init.c | 7 +++++++ > include/asm-generic/mshyperv.h | 2 ++ > 2 files changed, 9 insertions(+) Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
