Re: [PATCH v26 05/17] vfio: Add VM state change handler to know state of VM

From: Cornelia Huck <cohuck@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: cjia@nvidia.com, aik@ozlabs.ru, Zhengxiao.zx@Alibaba-inc.com,
	shuangtai.tst@alibaba-inc.com, qemu-devel@nongnu.org,
	peterx@redhat.com, Kirti Wankhede <kwankhede@nvidia.com>,
	eauger@redhat.com, yi.l.liu@intel.com, quintela@redhat.com,
	ziye.yang@intel.com, armbru@redhat.com, mlevitsk@redhat.com,
	pasic@linux.ibm.com, felipe@nutanix.com, zhi.a.wang@intel.com,
	kevin.tian@intel.com, yan.y.zhao@intel.com, dgilbert@redhat.com,
	changpeng.liu@intel.com, eskultet@redhat.com, Ken.Xue@amd.com,
	jonathan.davies@nutanix.com, pbonzini@redhat.com
Subject: Re: [PATCH v26 05/17] vfio: Add VM state change handler to know state of VM
Date: Tue, 20 Oct 2020 12:23:20 +0200	[thread overview]
Message-ID: <20201020122320.675dbcd4.cohuck@redhat.com> (raw)
In-Reply-To: <20201019115136.70c96820@w520.home>

On Mon, 19 Oct 2020 11:51:36 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Sun, 18 Oct 2020 23:13:39 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
> > <snip>
> >   
> > >>>> +vfio_migration_set_state(char *name, uint32_t state) " (%s) state %d"
> > >>>> +vfio_vmstate_change(char *name, int running, const char *reason, uint32_t dev_state) " (%s) running %d reason %s device state %d"
> > >>>> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> > >>>> index 8275c4c68f45..25e3b1a3b90a 100644
> > >>>> --- a/include/hw/vfio/vfio-common.h
> > >>>> +++ b/include/hw/vfio/vfio-common.h
> > >>>> @@ -29,6 +29,7 @@
> > >>>>    #ifdef CONFIG_LINUX
> > >>>>    #include <linux/vfio.h>
> > >>>>    #endif
> > >>>> +#include "sysemu/sysemu.h"
> > >>>>    
> > >>>>    #define VFIO_MSG_PREFIX "vfio %s: "
> > >>>>    
> > >>>> @@ -119,6 +120,9 @@ typedef struct VFIODevice {
> > >>>>        unsigned int flags;
> > >>>>        VFIOMigration *migration;
> > >>>>        Error *migration_blocker;
> > >>>> +    VMChangeStateEntry *vm_state;
> > >>>> +    uint32_t device_state;
> > >>>> +    int vm_running;    
> > >>>
> > >>> Could these be placed in VFIOMigration?  Thanks,
> > >>>       
> > >>
> > >> I think device_state should be part of VFIODevice since its about device
> > >> rather than only related to migration, others can be moved to VFIOMigration.    
> > > 
> > > But these are only valid when migration is supported and thus when
> > > VFIOMigration exists.  Thanks,
> > >     
> > 
> > Even though it is used when migration is supported, its device's attribute.  
> 
> device_state is a local copy of the migration region register, so it
> serves no purpose when a migration region is not present.  In fact the
> initial value would indicate the device is stopped, which is incorrect.
> vm_running is never initialized and cannot be set other than through a
> migration region update of device_state, so at least two of these
> values show incorrect state when migration is not supported by the
> device.  vm_state is unused when migration isn't present, so if nothing
> else the pointer here is wasteful.  It's not clear to me what
> justification is being presented here as a "device's attribute",
> supporting migration as indicated by a non-NULL migration pointer is
> also a device attribute and these are attributes further defining the
> state of that support.

Agreed. Also, it is not obvious from the naming that 'device_state' is
related to migration, and it is easy to assume that this field is
useful even in the non-migration case. Moving it would solve that
problem.