linux-kernel - Re: [PATCH] iommu/amd: Fix loop timeout issue in iommu_ga_log

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Date:   Mon, 31 Jan 2022 17:06:03 +0000
From:   John Garry <john.garry@...wei.com>
To:     Joerg Roedel <joro@...tes.org>, <iommu@...ts.linux-foundation.org>
CC:     Joerg Roedel <jroedel@...e.de>, <linux-kernel@...r.kernel.org>,
        "Maxim Levitsky" <mlevitsk@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] iommu/amd: Fix loop timeout issue in
 iommu_ga_log_enable()

On 31/01/2022 16:17, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> The polling loop for the register change in iommu_ga_log_enable() needs
> to have a udelay() in it.  Otherwise the CPU might be faster than the
> IOMMU hardware and wrongly trigger the WARN_ON() further down the code
> stream.
> 
> Fixes: 8bda0cfbdc1a ("iommu/amd: Detect and initialize guest vAPIC log")
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>   drivers/iommu/amd/init.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index dc338acf3338..d2e09d53851f 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -834,6 +834,7 @@ static int iommu_ga_log_enable(struct amd_iommu *iommu)
>   		status = readl(iommu->mmio_base + MMIO_STATUS_OFFSET);
>   		if (status & (MMIO_STATUS_GALOG_RUN_MASK))
>   			break;
> +		udelay(1);

Maybe readl_relaxed_poll_timeout_atomic() could be used instead

Thanks,
John

>   	}
>   
>   	if (WARN_ON(i >= LOOP_TIMEOUT))
>