linux-kernel - Re: [PATCH v5 3/6] iommu/amd: Modify set_dte

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Message-ID: <2cca938f-9229-435a-a038-c431fcff0622@amd.com>
Date: Wed, 16 Oct 2024 12:12:28 +0700
From: "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To: Jason Gunthorpe <jgg@...dia.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 v5 3/6] iommu/amd: Modify set_dte_entry() to use 256-bit
 DTE helpers

On 10/7/2024 9:05 PM, Jason Gunthorpe wrote:
> On Mon, Oct 07, 2024 at 04:13:50AM +0000, Suravee Suthikulpanit wrote:
>> +static void make_clear_dte(struct amd_iommu *iommu, struct dev_table_entry *dte,
>> +			   struct dev_table_entry *new)
>> +{
>> +	new->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)
>> +		new->data[0] |= DTE_FLAG_TV;
> 
> It would be nice to have a comment here..
I am moving this check. See description below...

> clear_dte() must create a blocking configuration as several callers
> depend on that.

Right, I missed that. I'll rework this function.

> Why is blocking with TV=1,Mode=0,IW=0,IR=0 used sometimes but
> sometimes TV=0 is used instead?

Originally, when DTE[Mode]=0, the TV bit is set.

Then, the commit b9f0043e1ea6 "iommu/amd: Set translation valid bit only 
when IO page tables are in use" clears the TV ONLY when running on 
SNP-enabled system. We didn't clear the bit for all cases since there 
was a concern whether it would cause regression on older platforms.

However, I am considering clearing the TV flag for all cases to simplify 
the logic since it should not violate the spec.

>> +	/* Need to preserve interrupt remapping information in DTE[128:255] */
>> +	new->data128[1] = dte->data128[1];
> 
> It doesn't need to preserve.. write_dte_upper128() does the
> preservation automatically under the right lock. Any bits in
> DTE_DATA2_INTR_MASK should be 0 for the input DTE because they will be
> ignored by the masking:
> 
> +               new->data[2] &= ~DTE_DATA2_INTR_MASK;
> +               new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK | DTE_DATA2_RESV_MASK);
> 
> Also this shouldn't preserve the top Guest related 64 bit for a 'clear
> dte' either.
> 
> So, I think this can just be
> 
>     new->data128[1] = 0;
> 
> ?

Good point. I'll clean this up.

Thanks,
Suravee