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

[Vasant Hegde <vasant.hegde@amd.com>]

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

> +{
> +	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?

-Vasant