Re: [RFC][PATCH 09/12] x86,amd_iommu: Replace cmpxchg_double()

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2022-12-19 at 16:35 +0100, Peter Zijlstra wrote:
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  drivers/iommu/amd/amd_iommu_types.h |    9 +++++++--
>  drivers/iommu/amd/iommu.c           |   10 ++++------
>  2 files changed, 11 insertions(+), 8 deletions(-)
> 
> --- a/drivers/iommu/amd/amd_iommu_types.h
> +++ b/drivers/iommu/amd/amd_iommu_types.h
> @@ -979,8 +979,13 @@ union irte_ga_hi {
>  };
>  
>  struct irte_ga {
> -	union irte_ga_lo lo;
> -	union irte_ga_hi hi;
> +	union {
> +		struct {
> +			union irte_ga_lo lo;
> +			union irte_ga_hi hi;
> +		};
> +		u128 irte;
> +	};
>  };
>  
>  struct irq_2_irte {
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -2992,10 +2992,10 @@ static int alloc_irq_index(struct amd_io
>  static int modify_irte_ga(struct amd_iommu *iommu, u16 devid, int index,
>  			  struct irte_ga *irte, struct amd_ir_data *data)
>  {
> -	bool ret;
>  	struct irq_remap_table *table;
> -	unsigned long flags;
>  	struct irte_ga *entry;
> +	unsigned long flags;
> +	u128 old;
>  
>  	table = get_irq_table(iommu, devid);
>  	if (!table)
> @@ -3006,16 +3006,14 @@ static int modify_irte_ga(struct amd_iom
>  	entry = (struct irte_ga *)table->table;
>  	entry = &entry[index];
>  
> -	ret = cmpxchg_double(&entry->lo.val, &entry->hi.val,
> -			     entry->lo.val, entry->hi.val,
> -			     irte->lo.val, irte->hi.val);
>  	/*
>  	 * We use cmpxchg16 to atomically update the 128-bit IRTE,
>  	 * and it cannot be updated by the hardware or other processors
>  	 * behind us, so the return value of cmpxchg16 should be the
>  	 * same as the old value.

The above comment seems to have already been out of date but could be
updated to say cmpxchg128 instead of cmxchg16 anyway.

>  	 */
> -	WARN_ON(!ret);
> +	old = entry->irte;
> +	WARN_ON(!try_cmpxchg128(&entry->irte, &old, irte->irte));
>  
>  	if (data)
>  		data->ref = entry;
> 
>