linux-kernel - Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in intel_pasid_get

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Date: Thu, 23 May 2024 21:24:36 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Uros Bizjak <ubizjak@...il.com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Cc: baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in
 intel_pasid_get_entry()

On 2024/5/22 16:26, Uros Bizjak wrote:
> Use try_cmpxchg64() instead of cmpxchg64 (*ptr, old, new) != old in
> intel_pasid_get_entry().  cmpxchg returns success in ZF flag, so
> this change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> Cc: Lu Baolu <baolu.lu@...ux.intel.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Robin Murphy <robin.murphy@....com>
> ---
>   drivers/iommu/intel/pasid.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
> index abce19e2ad6f..9bf45bc4b967 100644
> --- a/drivers/iommu/intel/pasid.c
> +++ b/drivers/iommu/intel/pasid.c
> @@ -146,6 +146,8 @@ static struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
>   retry:
>   	entries = get_pasid_table_from_pde(&dir[dir_index]);
>   	if (!entries) {
> +		u64 tmp;
> +
>   		entries = iommu_alloc_page_node(info->iommu->node, GFP_ATOMIC);
>   		if (!entries)
>   			return NULL;
> @@ -156,8 +158,9 @@ static struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
>   		 * clear. However, this entry might be populated by others
>   		 * while we are preparing it. Use theirs with a retry.
>   		 */
> -		if (cmpxchg64(&dir[dir_index].val, 0ULL,
> -			      (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
> +		tmp = 0ULL;

nit: can this simply be
		tmp = 0;
?

> +		if (!try_cmpxchg64(&dir[dir_index].val, &tmp,
> +				   (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {

Above change will cause a dead loop during boot. It should be

		if (try_cmpxchg64(&dir[dir_index].val, &tmp,
				  (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {

>   			iommu_free_page(entries);
>   			goto retry;
>   		}

Best regards,
baolu