Re: [PATCH] x86/hyperv: Restore VP assist page after cpu offlining/onlining

[Wei Liu]

On Thu, Nov 03, 2022 at 08:06:01PM +0100, Vitaly Kuznetsov wrote:
> Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
> to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
> 'if (*hvp)' condition. This works for root partition as hv_cpu_die()
> does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
> non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
> for them. This causes VP assist page to remain unset after CPU
> offline/online cycle:
> 
> $ rdmsr -p 24 0x40000073
>   10212f001
> $ echo 0 > /sys/devices/system/cpu/cpu24/online
> $ echo 1 > /sys/devices/system/cpu/cpu24/online
> $ rdmsr -p 24 0x40000073
>   0
> 
> Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
> hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
> pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
> NULL (and it's also NULL initially).
> 
> Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
> present a (potential) issue for KVM. While Hyper-V uses
> CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses CPUHP_AP_KVM_STARTING
> which comes earlier in CPU teardown sequence. It is theoretically
> possible that Enlightened VMCS is still in use. It is unclear if the
> issue is real and if using KVM with Hyper-V root partition is even
> possible.
> 
> While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
> 
> Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist page MSR")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Applied to hyperv-fixes. Thanks.