linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Remove commented-out code

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Date:   Mon, 12 Jun 2023 11:15:09 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Remove commented-out code

On Fri, Jun 09, 2023 at 02:05:14PM +0800, Lu Baolu wrote:
> These lines of code were commented out when they were first added in commit
> ba39592764ed ("Intel IOMMU: Intel IOMMU driver"). We do not want to restore
> them because the VT-d spec has deprecated the read/write draining hit.
> 
> VT-d spec (section 11.4.2):
> "
>  Hardware implementation with Major Version 2 or higher (VER_REG), always
>  performs required drain without software explicitly requesting a drain in
>  IOTLB invalidation. This field is deprecated and hardware  will always
>  report it as 1 to maintain backward compatibility with software.
> "
> 
> Remove the code to make the code cleaner.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>

> ---
>  drivers/iommu/intel/iommu.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 4c0b7424c45e..e5c111ff4dd9 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1312,15 +1312,7 @@ static void __iommu_flush_iotlb(struct intel_iommu *iommu, u16 did,
>  			iommu->name, type);
>  		return;
>  	}
> -	/* Note: set drain read/write */
> -#if 0
> -	/*
> -	 * This is probably to be super secure.. Looks like we can
> -	 * ignore it without any impact.
> -	 */
> -	if (cap_read_drain(iommu->cap))
> -		val |= DMA_TLB_READ_DRAIN;
> -#endif
> +
>  	if (cap_write_drain(iommu->cap))
>  		val |= DMA_TLB_WRITE_DRAIN;
>  
> -- 
> 2.34.1
>