linux-kernel - Re: [PATCH v3 2/6] iommu/amd: Do not use flush-queue when NpCache is on

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Date:   Tue, 15 Jun 2021 14:08:16 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nadav Amit <nadav.amit@...il.com>, Joerg Roedel <joro@...tes.org>
Cc:     Nadav Amit <namit@...are.com>, Will Deacon <will@...nel.org>,
        Jiajun Cao <caojiajun@...are.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] iommu/amd: Do not use flush-queue when NpCache is
 on

On 2021-06-07 19:25, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> Do not use flush-queue on virtualized environments, where the NpCache
> capability of the IOMMU is set. This is required to reduce
> virtualization overheads.
> 
> This change follows a similar change to Intel's VT-d and a detailed
> explanation as for the rationale is described in commit 29b32839725f
> ("iommu/vt-d: Do not use flush-queue when caching-mode is on").
> 
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Jiajun Cao <caojiajun@...are.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Lu Baolu <baolu.lu@...ux.intel.com>
> Cc: iommu@...ts.linux-foundation.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>   drivers/iommu/amd/init.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index d006724f4dc2..ba3b76ed776d 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -1850,8 +1850,13 @@ static int __init iommu_init_pci(struct amd_iommu *iommu)
>   	if (ret)
>   		return ret;
>   
> -	if (iommu->cap & (1UL << IOMMU_CAP_NPCACHE))
> +	if (iommu->cap & (1UL << IOMMU_CAP_NPCACHE)) {
> +		if (!amd_iommu_unmap_flush)
> +			pr_warn_once("IOMMU batching is disabled due to virtualization");

Nit: you can just use pr_warn() (or arguably pr_info()) since the 
explicit conditions already only match once. Speaking of which, it might 
be better to use amd_iommu_np_cache instead, since other patches are 
planning to clean up the last remnants of amd_iommu_unmap_flush.

Robin.

> +
>   		amd_iommu_np_cache = true;
> +		amd_iommu_unmap_flush = true;
> +	}
>   
>   	init_iommu_perf_ctr(iommu);
>   
>