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Message-ID: <3088ea5ffdfd095ca67265ed711e8d2bc188b362.camel@linux.ibm.com>
Subject: Re: [RFC][PATCH 09/12] x86,amd_iommu: Replace cmpxchg_double()
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        torvalds@linux-foundation.org
Cc:     corbet@lwn.net, will@kernel.org, boqun.feng@gmail.com,
        mark.rutland@arm.com, catalin.marinas@arm.com, dennis@kernel.org,
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        baolu.lu@linux.intel.com, Arnd Bergmann <arnd@arndb.de>,
        penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com,
        Andrew Morton <akpm@linux-foundation.org>, vbabka@suse.cz,
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        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
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        linux-crypto@vger.kernel.org, iommu@lists.linux.dev,
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Date:   Mon, 19 Dec 2022 17:47:47 +0100
In-Reply-To: <20221219154119.419176389@infradead.org>
References: <20221219153525.632521981@infradead.org>
         <20221219154119.419176389@infradead.org>
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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(-)
>=20
> --- a/drivers/iommu/amd/amd_iommu_types.h
> +++ b/drivers/iommu/amd/amd_iommu_types.h
> @@ -979,8 +979,13 @@ union irte_ga_hi {
>  };
> =20
>  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;
> +	};
>  };
> =20
>  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;
> =20
>  	table =3D get_irq_table(iommu, devid);
>  	if (!table)
> @@ -3006,16 +3006,14 @@ static int modify_irte_ga(struct amd_iom
>  	entry =3D (struct irte_ga *)table->table;
>  	entry =3D &entry[index];
> =20
> -	ret =3D 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 =3D entry->irte;
> +	WARN_ON(!try_cmpxchg128(&entry->irte, &old, irte->irte));
> =20
>  	if (data)
>  		data->ref =3D entry;
>=20
>=20