Re: [Xen-devel] [PATCH] x86/AMD-Vi: Fold exit paths of {enable, disable}_iommu()

["Woods, Brian" <Brian.Woods@amd.com>]

On Mon, Aug 12, 2019 at 06:52:05PM +0100, Andy Cooper wrote:
> ... to avoid having multiple spin_unlock_irqrestore() calls.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Acked-by: Brian Woods <brian.woods@amd.com>

> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Wei Liu <wl@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> CC: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> CC: Brian Woods <brian.woods@amd.com>
> 
> Interestingly GCC 6.3 managed to fold disable_iommu() automatically.  There is
> some partial folding for enable_iommu() (insofar as there is only a single
> call to _spin_unlock_irqrestore emitted), but this delta yeilds
> 
>   add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-20 (-20)
>   Function                                     old     new   delta
>   enable_iommu                                1844    1824     -20
>   Total: Before=3340299, After=3340279, chg -0.00%
> 
> which means that something wasn't done automatically.
> 
> Noticed while investigating the S3 regression.
> ---
>  xen/drivers/passthrough/amd/iommu_init.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/xen/drivers/passthrough/amd/iommu_init.c b/xen/drivers/passthrough/amd/iommu_init.c
> index bb9f33e264..bb5a3e57c9 100644
> --- a/xen/drivers/passthrough/amd/iommu_init.c
> +++ b/xen/drivers/passthrough/amd/iommu_init.c
> @@ -899,11 +899,8 @@ static void enable_iommu(struct amd_iommu *iommu)
>  
>      spin_lock_irqsave(&iommu->lock, flags);
>  
> -    if ( iommu->enabled )
> -    {
> -        spin_unlock_irqrestore(&iommu->lock, flags); 
> -        return;
> -    }
> +    if ( unlikely(iommu->enabled) )
> +        goto out;
>  
>      amd_iommu_erratum_746_workaround(iommu);
>  
> @@ -957,6 +954,8 @@ static void enable_iommu(struct amd_iommu *iommu)
>          amd_iommu_flush_all_caches(iommu);
>  
>      iommu->enabled = 1;
> +
> + out:
>      spin_unlock_irqrestore(&iommu->lock, flags);
>  }
>  
> @@ -966,11 +965,8 @@ static void disable_iommu(struct amd_iommu *iommu)
>  
>      spin_lock_irqsave(&iommu->lock, flags);
>  
> -    if ( !iommu->enabled )
> -    {
> -        spin_unlock_irqrestore(&iommu->lock, flags);
> -        return;
> -    }
> +    if ( unlikely(!iommu->enabled) )
> +        goto out;
>  
>      if ( !iommu->ctrl.int_cap_xt_en )
>          amd_iommu_msi_enable(iommu, IOMMU_CONTROL_DISABLED);
> @@ -988,6 +984,7 @@ static void disable_iommu(struct amd_iommu *iommu)
>  
>      iommu->enabled = 0;
>  
> + out:
>      spin_unlock_irqrestore(&iommu->lock, flags);
>  }
>  
> -- 
> 2.11.0
> 

-- 
Brian Woods

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