linux-kernel - Re: [PATCH v5 2/6] iommu/amd: Introduce helper function to update 256-bit DTE

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Message-ID: <cc44a5ca-ddf9-ca7a-93f8-38bf26ac1f1f@gmail.com>
Date: Mon, 7 Oct 2024 16:42:30 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Cc: joro@...tes.org, robin.murphy@....com, vasant.hegde@....com,
 jgg@...dia.com, kevin.tian@...el.com, jon.grimm@....com,
 santosh.shukla@....com, pandoh@...gle.com, kumaranand@...gle.com
Subject: Re: [PATCH v5 2/6] iommu/amd: Introduce helper function to update
 256-bit DTE



On 7. 10. 24 06:13, Suravee Suthikulpanit wrote:

> +
>   /****************************************************************************
>    *
>    * Helper functions
>    *
>    ****************************************************************************/
>   
> +static void write_dte_upper128(struct dev_table_entry *ptr, struct dev_table_entry *new)
> +{
> +	struct dev_table_entry old = {};
> +
> +	do {
> +		old.data128[1] = ptr->data128[1];
> +		new->data[2] &= ~DTE_DATA2_INTR_MASK;
> +		new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK | DTE_DATA2_RESV_MASK);
> +	} while (!try_cmpxchg128(&ptr->data128[1], &old.data128[1], new->data128[1]));

Please note that try_cmpxchg inherently updates &old.data128[1] above on 
failure. There is no need to update value again in the loop.

Please also note that the value from ptr->data128[1] should be read 
using READ_ONCE() to prevent compiler from merging, refetching or 
reordering the read. Currently, there is no READ_ONCE() implemented for 
__int128, so something like the attached patch should be used.

Based on the above, the loop should be rewritten as:

	old.data128[1] = READ_ONCE(ptr->data128[1]);
	do {
		new->data[2] &= ~DTE_DATA2_INTR_MASK;
		new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK | DTE_DATA2_RESV_MASK);
	} while (!try_cmpxchg128(&ptr->data128[1], &old.data128[1], 
new->data128[1]));

> +}
> +
> +static void write_dte_lower128(struct dev_table_entry *ptr, struct dev_table_entry *new)
> +{
> +	struct dev_table_entry old = {};
> +
> +	/*
> +	 * Need to preserve DTE[96:106], which can be set by information in IVRS table.
> +	 * See set_dev_entry_from_acpi().
> +	 */
> +	new->data[1] |= ptr->data[1] & DTE_FLAG_MASK;
> +
> +	do {
> +		old.data128[0] = ptr->data128[0];
> +	} while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], new->data128[0]));

And this one as:

	old.data128[0] = READ_ONCE(ptr->data128[0]);
	do {
	} while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], 
new->data128[0]));

Best regards,
Uros.
View attachment "p.diff.txt" of type "text/plain" (1411 bytes)