Re: [PATCH v3 4/4] KVM: arm64: Allow no running vcpu on saving vgic3 pending table

[Zenghui Yu <zenghui.yu@linux.dev>]

On 2023/1/27 07:54, Gavin Shan wrote:
> We don't have a running VCPU context to save vgic3 pending table due
> to KVM_DEV_ARM_VGIC_{GRP_CTRL, SAVE_PENDING_TABLES} command on KVM
> device "kvm-arm-vgic-v3". The unknown case is caught by kvm-unit-tests.
> 
>    # ./kvm-unit-tests/tests/its-pending-migration
>    WARNING: CPU: 120 PID: 7973 at arch/arm64/kvm/../../../virt/kvm/kvm_main.c:3325 \
>    mark_page_dirty_in_slot+0x60/0xe0
>     :
>    mark_page_dirty_in_slot+0x60/0xe0
>    __kvm_write_guest_page+0xcc/0x100
>    kvm_write_guest+0x7c/0xb0
>    vgic_v3_save_pending_tables+0x148/0x2a0
>    vgic_set_common_attr+0x158/0x240
>    vgic_v3_set_attr+0x4c/0x5c
>    kvm_device_ioctl+0x100/0x160
>    __arm64_sys_ioctl+0xa8/0xf0
>    invoke_syscall.constprop.0+0x7c/0xd0
>    el0_svc_common.constprop.0+0x144/0x160
>    do_el0_svc+0x34/0x60
>    el0_svc+0x3c/0x1a0
>    el0t_64_sync_handler+0xb4/0x130
>    el0t_64_sync+0x178/0x17c
> 
> Use vgic_write_guest_lock() to save vgic3 pending table.
> 
> Reported-by: Zenghui Yu <yuzenghui@huawei.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  Documentation/virt/kvm/api.rst | 4 +++-
>  arch/arm64/kvm/vgic/vgic-v3.c  | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 40ada313faa3..07f07668995e 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8074,7 +8074,9 @@ NOTE: Multiple examples of using the backup bitmap: (1) save vgic/its
>  tables through command KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_SAVE_TABLES} on
>  KVM device "kvm-arm-vgic-its". (2) restore vgic/its tables through
>  command KVM_DEV_ARM_{VGIC_GRP_CTRL, ITS_RESTORE_TABLES} on KVM device
> -"kvm-arm-vgic-its". vgic3 LPI pending status is restored.
> +"kvm-arm-vgic-its". vgic3 LPI pending status is restored. (3) save
> +vgic3 pending table through KVM_DEV_ARM_VGIC_{GRP_CTRL, SAVE_PENDING_TABLES}
> +command on KVM device "kvm-arm-vgic-v3".

Can we summarize these 3 examples with something like: "when the guest
memory (pending tables, ITS tables, etc) is dirtied by the virtual GIC
or ITS, which is typically triggered by a userspace request (e.g.,
KVM_DEV_ARM_ITS_SAVE_TABLES) and doesn't require a running VCPU
context"? In case there will be more no-running-vcpu
kvm_write_guest_lock() cases in the VGIC emulation code in future and we
have to extend the documentation..

But I don't have objection to your writing and the whole series looks
good.

Thanks,
Zenghui