linux-kernel - Re: [PATCH v4 07/11] KVM: arm/arm64: vgic-its: Always attempt to save/restore device and collection tables

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Date:   Wed, 18 Oct 2017 00:15:02 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        marc.zyngier@....com, peter.maydell@...aro.org,
        andre.przywara@....com, wanghaibin.wang@...wei.com,
        wu.wubin@...wei.com, drjones@...hat.com, wei@...hat.com
Subject: Re: [PATCH v4 07/11] KVM: arm/arm64: vgic-its: Always attempt to
 save/restore device and collection tables

On Tue, Oct 17, 2017 at 09:10:05AM +0200, Eric Auger wrote:
> In case the device table save fails, we currently do not
> attempt to save the collection table. However it may
> happen that the device table fails because the structures
> in memory are inconsistent with device GITS_BASER however
> this does not mean collection backup can't and shouldn't
> be performed. Same must happen on restore path.
> 
> Without this patch, after a reset and early state backup,
> the device table restore may fail due to L1 entry not valid.
> If we don't restore the collection table the guest does
> not reboot properly.

I don't really understand.  After the previous patches, why would a
properly configured ITS return an error in its_save_device_tables?

If that's not possible, are we not trying to support partially migrating
half-way broken state, and is that something we care about?
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> ---
> 
> candidate to be CC'ed stable
> ---
>  virt/kvm/arm/vgic/vgic-its.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index e18f1e4..1c3e83f 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -2381,12 +2381,9 @@ static int vgic_its_save_tables_v0(struct vgic_its *its)
>  	}
>  
>  	ret = vgic_its_save_device_tables(its);
> -	if (ret)
> -		goto out;
>  
> -	ret = vgic_its_save_collection_table(its);
> +	ret |= vgic_its_save_collection_table(its);

What if the two functions return two different error codes, is the
binary OR of these error codes going to tell userspace anything
meaningful?


>  
> -out:
>  	unlock_all_vcpus(kvm);
>  	mutex_unlock(&its->its_lock);
>  	mutex_unlock(&kvm->lock);
> @@ -2413,11 +2410,9 @@ static int vgic_its_restore_tables_v0(struct vgic_its *its)
>  	}
>  
>  	ret = vgic_its_restore_collection_table(its);
> -	if (ret)
> -		goto out;
>  
> -	ret = vgic_its_restore_device_tables(its);
> -out:
> +	ret |= vgic_its_restore_device_tables(its);
> +
>  	unlock_all_vcpus(kvm);
>  	mutex_unlock(&its->its_lock);
>  	mutex_unlock(&kvm->lock);
> -- 
> 2.5.5
> 

Thanks,
-Christoffer