Re: [PATCH v2] iommu/amd: Make PerfOpt compulsory

[Mario Limonciello <superm1@kernel.org>]

On 9/30/26 04:38, Vasant Hegde wrote:
> Mario,
> 
> On 9/29/2026 7:04 AM, Mario Limonciello (AMD) wrote:
>> PerfOpt is only a feature usable by integrated GPUs and only in identity
>> mode.  Instead of leaving a policy knob in amdgpu, just turn it on when
>> an integrated GPU is in identity.
>>
>> This drops quite a bit of compatibility glue.  There was a refcounting
>> system, exported symbols, and device attach/detach logic.  By just setting
>> it immediately it's a lot more straightforward.
>>
>> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
>> Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
>> ---
>> v2:
>>   * Move WARN_ON to amd_iommu_perfopt_clear()
>>   * Make amd_iommu_perfopt_clear() void
>>   * Drop unnecessary cleanup/create paths that unset feature
>>   * Enable the feature in IOMMU if supported (per guidance in IVRS spec)
>>   * Use existing check_feature() helper instead
>> ---
> 
> 
> .../...
> 
>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>> index c9b28e5e582ea..a1adb40d5b79d 100644
>> --- a/drivers/iommu/amd/iommu.c
>> +++ b/drivers/iommu/amd/iommu.c
>> @@ -2345,6 +2345,35 @@ static void pdom_detach_iommu(struct amd_iommu *iommu,
>>   	spin_unlock_irqrestore(&pdom->lock, flags);
>>   }
>>   
>> +/*
>> + * Program or clear the per-IOMMU PerfOpt enable bit
>> + * This allows a privileged internal GPU to be able to bypass IOMMU when
>> + * in identity mode.
>> + */
>> +int amd_iommu_perfopt_set(struct amd_iommu *iommu, bool enable)
> 
> Please move this to init.c file

Thanks, good suggestion.

> 
>> +{
>> +	u32 old, val, readback;
>> +
>> +	if (!check_feature(FEATURE_PERF_OPT))
>> +		return enable ? -ENODEV : 0;
>> +
>> +	old = readl(iommu->mmio_base + MMIO_PERF_OPT_OFFSET);
>> +	if (old == U32_MAX)
>> +		return -EIO;
>> +
>> +	val = enable ? old | PERF_OPT_EN : old & ~PERF_OPT_EN;
>> +	if (val != old)
>> +		writel(val, iommu->mmio_base + MMIO_PERF_OPT_OFFSET);
>> +	readback = readl(iommu->mmio_base + MMIO_PERF_OPT_OFFSET);
>> +	if (readback == U32_MAX ||
>> +	    (readback & PERF_OPT_EN) != (val & PERF_OPT_EN)) {
>> +		dev_err(iommu->iommu.dev, "Failed to set PerfOpt to %d\n", enable);
>> +		return -EIO;
>> +	}
>> +
> 
> May be check return value before enabling dev_data->perfopt?

I thought about this.

Let me know what you think of the approach of clearing the bit in 
amd_iommu_efr on failures within amd_iommu_perfopt_set().

That would mean that dev_data->perfopt doesn't get enabled later.