linux-kernel - Re: [PATCH] iommu/vt-d: Handle volatile descriptor status read

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Date: Tue, 18 Jun 2024 10:54:18 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>, Lu Baolu
	<baolu.lu@...ux.intel.com>, LKML <linux-kernel@...r.kernel.org>,
	<iommu@...ts.linux.dev>
CC: "Tian, Kevin" <kevin.tian@...el.com>, <sanjay.k.kumar@...el.com>
Subject: Re: [PATCH] iommu/vt-d: Handle volatile descriptor status read

On 2024/6/8 01:38, Jacob Pan wrote:
> Queued invalidation wait descriptor status is volatile in that IOMMU hardware
> writes the data upon completion.
> 
> Use READ_ONCE() to prevent compiler optimizations which ensures memory
> reads every time. As a side effect, READ_ONCE() also enforces strict types and
> may add an extra instruction. But it should not have negative
> performance impact since we use cpu_relax anyway and the extra time(by
> adding an instruction) may allow IOMMU HW request cacheline ownership easier.
> 
> e.g. gcc 12.3
> BEFORE:
> 	81 38 ad de 00 00       cmpl   $0x2,(%rax)
> 
> AFTER (with READ_ONCE())
>      772f:       8b 00                   mov    (%rax),%eax
>      7731:       3d ad de 00 00          cmp    $0x2,%eax //status data is 32 bit
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>   drivers/iommu/intel/dmar.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> index 304e84949ca7..1c8d3141cb55 100644
> --- a/drivers/iommu/intel/dmar.c
> +++ b/drivers/iommu/intel/dmar.c
> @@ -1446,7 +1446,7 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
>   	 */
>   	writel(qi->free_head << shift, iommu->reg + DMAR_IQT_REG);
>   
> -	while (qi->desc_status[wait_index] != QI_DONE) {
> +	while (READ_ONCE(qi->desc_status[wait_index]) != QI_DONE) {
>   		/*
>   		 * We will leave the interrupts disabled, to prevent interrupt
>   		 * context to queue another cmd while a cmd is already submitted

Reviewed-by: Yi Liu <yi.l.liu@...el.com>

-- 
Regards,
Yi Liu