linux-kernel - Re: [PATCH v4 5/6] iommu/amd: Modify clear_dte

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Message-ID: <20240926195423.GR9417@nvidia.com>
Date: Thu, 26 Sep 2024 16:54:23 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, joro@...tes.org,
	robin.murphy@....com, vasant.hegde@....com, kevin.tian@...el.com,
	jon.grimm@....com, santosh.shukla@....com, pandoh@...gle.com,
	kumaranand@...gle.com
Subject: Re: [PATCH v4 5/6] iommu/amd: Modify clear_dte_entry() to avoid
 in-place update

On Mon, Sep 16, 2024 at 05:18:04PM +0000, Suravee Suthikulpanit wrote:

> -static void clear_dte_entry(struct amd_iommu *iommu, u16 devid)
> +static void clear_dte_entry(struct amd_iommu *iommu, struct iommu_dev_data *dev_data)
>  {
> -	struct dev_table_entry *dev_table = get_dev_table(iommu);
> +	struct dev_table_entry new;
> +	struct dev_table_entry *dte = &get_dev_table(iommu)[dev_data->devid];
> +
> +	/*
> +	 * Need to preserve DTE[96:106] because certain fields are
> +	 * programmed using value in IVRS table from early init phase.
> +	 */
> +	new.data[0] = DTE_FLAG_V;
>  
> -	/* remove entry from the device table seen by the hardware */
> -	dev_table[devid].data[0]  = DTE_FLAG_V;
> +	/* Apply erratum 63 */
> +	if (FIELD_GET(DTE_SYSMGT_MASK, dte->data[1]) == 0x01)
> +		new.data[0] |= BIT_ULL(DEV_ENTRY_IW);
>  
>  	if (!amd_iommu_snp_en)
> -		dev_table[devid].data[0] |= DTE_FLAG_TV;
> +		new.data[0] |= DTE_FLAG_TV;
> +
> +	/* Need to preserve DTE[96:106] */
> +	new.data[1] = dte->data[1] & DTE_FLAG_MASK;
>  
> -	dev_table[devid].data[1] &= DTE_FLAG_MASK;
> +	/* Need to preserve interrupt remapping information in DTE[128:255] */
> +	new.data128[1] = dte->data128[1];

It doesn't, update_dte256() does this automatically. Just leave it
zero here.

> -	amd_iommu_apply_erratum_63(iommu, devid);
> +	update_dte256(iommu, dev_data, &new);
>  }

I suggest you change this slightly so the flow is more like

make_clear_dte(..., struct dev_table_entry *entry) {..}

Which would have most of the above. Then:

clear_dte_entry()
{
    struct dev_table_entry target;

    make_clear_dte(.., &target);
    update_dte256(iommu, dev_data, &new);
}

And then in the prior patches you can write like:

static void make_dte_gcr3_table(struct amd_iommu *iommu,
                              struct iommu_dev_data *dev_data,
                              struct dev_table_entry *target)
{
    make_clear_dte(.., &target);
    ...
}

And drop all the wild masking:

+       /* First mask out possible old values for GCR3 table */
+       tmp = DTE_GCR3_VAL_A(~0ULL) << DTE_GCR3_SHIFT_A;
+       target->data[0] &= ~tmp;
+       tmp = DTE_GCR3_VAL_B(~0ULL) << DTE_GCR3_SHIFT_B;
+       tmp |= DTE_GCR3_VAL_C(~0ULL) << DTE_GCR3_SHIFT_C;
+       target->data[1] &= ~tmp;

Since make_clear_dte() already zero'd these fields.

Jason