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Message-ID: <933407a1-7743-095f-0b60-247d5078519f@linux.ibm.com>
Date:   Mon, 9 Jan 2023 15:24:35 -0500
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Subject: Re: [PATCH 1/2] KVM: async kvm_destroy_vm for vfio devices
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     alex.williamson@redhat.com, pbonzini@redhat.com, cohuck@redhat.com,
        farman@linux.ibm.com, pmorel@linux.ibm.com,
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        imbrenda@linux.ibm.com, david@redhat.com, akrowiak@linux.ibm.com,
        jjherne@linux.ibm.com, pasic@linux.ibm.com,
        zhenyuw@linux.intel.com, zhi.a.wang@intel.com,
        linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <20230109201037.33051-1-mjrosato@linux.ibm.com>
 <20230109201037.33051-2-mjrosato@linux.ibm.com> <Y7x1Z64g4H0kWOo/@nvidia.com>
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From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <Y7x1Z64g4H0kWOo/@nvidia.com>
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On 1/9/23 3:13 PM, Jason Gunthorpe wrote:
> On Mon, Jan 09, 2023 at 03:10:36PM -0500, Matthew Rosato wrote:
>> Currently it is possible that the final put of a KVM reference comes from
>> vfio during its device close operation.  This occurs while the vfio group
>> lock is held; however, if the vfio device is still in the kvm device list,
>> then the following call chain could result in a deadlock:
>>
>> kvm_put_kvm
>>  -> kvm_destroy_vm
>>   -> kvm_destroy_devices
>>    -> kvm_vfio_destroy
>>     -> kvm_vfio_file_set_kvm
>>      -> vfio_file_set_kvm
>>       -> group->group_lock/group_rwsem
>>
>> Avoid this scenario by adding kvm_put_kvm_async which will perform the
>> kvm_destroy_vm asynchronously if the refcount reaches 0.
>>
>> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
>> Reported-by: Alex Williamson <alex.williamson@redhat.com>
>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>> ---
>>  drivers/gpu/drm/i915/gvt/kvmgt.c  |  6 +++++-
>>  drivers/s390/crypto/vfio_ap_ops.c |  7 ++++++-
>>  include/linux/kvm_host.h          |  3 +++
>>  virt/kvm/kvm_main.c               | 22 ++++++++++++++++++++++
>>  4 files changed, 36 insertions(+), 2 deletions(-)
> 
> Why two patches?
> 
> It looks OK to me

Mentioned in the cover, the fixes: tag is different on the 2nd patch as the s390 PCI passthrough kvm_puts were added later soemtime after 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM").  The blamed commit for those changes also landed in a different release (6.0 vs 5.19).

But, now that you mention it, neither is an LTS so it probably doesn't matter all that much and could be squashed if preferred. 

> 
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Thanks!

> 
> Jason