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Subject: Re: [PATCH 2/2] iommu/amd: Remove performance counter
 pre-initialization test
To:     Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org
Cc:     joro@8bytes.org, will@kernel.org, jsnitsel@redhat.com,
        pmenzel@molgen.mpg.de, Jon.Grimm@amd.com,
        Tj <ml.linux@elloe.vision>,
        Alexander Monakov <amonakov@ispras.ru>,
        David Coe <david.coe@live.co.uk>,
        Shuah Khan <skhan@linuxfoundation.org>
References: <20210409085848.3908-1-suravee.suthikulpanit@amd.com>
 <20210409085848.3908-3-suravee.suthikulpanit@amd.com>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <6d53ded8-9ab8-7205-f17e-d0c315b770bd@linuxfoundation.org>
Date:   Fri, 9 Apr 2021 10:37:14 -0600
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On 4/9/21 2:58 AM, Suravee Suthikulpanit wrote:
> In early AMD desktop/mobile platforms (during 2013), when the IOMMU
> Performance Counter (PMC) support was first introduced in
> commit 30861ddc9cca ("perf/x86/amd: Add IOMMU Performance Counter
> resource management"), there was a HW bug where the counters could not
> be accessed. The result was reading of the counter always return zero.
> 
> At the time, the suggested workaround was to add a test logic prior
> to initializing the PMC feature to check if the counters can be programmed
> and read back the same value. This has been working fine until the more
> recent desktop/mobile platforms start enabling power gating for the PMC,
> which prevents access to the counters. This results in the PMC support
> being disabled unnecesarily.

unnecessarily

> 
> Unfortunatly, there is no documentation of since which generation

Unfortunately,

Rephrase suggestion:
Unfortunately, it is unclear when the PMC HW bug fixed.

> of hardware the original PMC HW bug was fixed. Although, it was fixed
> soon after the first introduction of the PMC. Base on this, we assume

Based

> that the buggy platforms are less likely to be in used, and it should

in use

> be relatively safe to remove this legacy logic.

> 
> Link: https://lore.kernel.org/linux-iommu/alpine.LNX.3.20.13.2006030935570.3181@monopod.intra.ispras.ru/
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201753
> Cc: Tj (Elloe Linux) <ml.linux@elloe.vision>
> Cc: Shuah Khan <skhan@linuxfoundation.org>
> Cc: Alexander Monakov <amonakov@ispras.ru>
> Cc: David Coe <david.coe@live.co.uk>
> Cc: Paul Menzel <pmenzel@molgen.mpg.de>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> ---
>   drivers/iommu/amd/init.c | 24 +-----------------------
>   1 file changed, 1 insertion(+), 23 deletions(-)
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index 648cdfd03074..247cdda5d683 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -1714,33 +1714,16 @@ static int __init init_iommu_all(struct acpi_table_header *table)
>   	return 0;
>   }
>   
> -static int iommu_pc_get_set_reg(struct amd_iommu *iommu, u8 bank, u8 cntr,
> -				u8 fxn, u64 *value, bool is_write);
> -
>   static void init_iommu_perf_ctr(struct amd_iommu *iommu)
>   {
> +	u64 val;
>   	struct pci_dev *pdev = iommu->dev;
> -	u64 val = 0xabcd, val2 = 0, save_reg = 0;
>   
>   	if (!iommu_feature(iommu, FEATURE_PC))
>   		return;
>   
>   	amd_iommu_pc_present = true;
>   
> -	/* save the value to restore, if writable */
> -	if (iommu_pc_get_set_reg(iommu, 0, 0, 0, &save_reg, false))
> -		goto pc_false;
> -
> -	/* Check if the performance counters can be written to */
> -	if ((iommu_pc_get_set_reg(iommu, 0, 0, 0, &val, true)) ||
> -	    (iommu_pc_get_set_reg(iommu, 0, 0, 0, &val2, false)) ||
> -	    (val != val2))
> -		goto pc_false;
> -
> -	/* restore */
> -	if (iommu_pc_get_set_reg(iommu, 0, 0, 0, &save_reg, true))
> -		goto pc_false;
> -
>   	pci_info(pdev, "IOMMU performance counters supported\n");
>   
>   	val = readl(iommu->mmio_base + MMIO_CNTR_CONF_OFFSET);
> @@ -1748,11 +1731,6 @@ static void init_iommu_perf_ctr(struct amd_iommu *iommu)
>   	iommu->max_counters = (u8) ((val >> 7) & 0xf);
>   
>   	return;
> -
> -pc_false:
> -	pci_err(pdev, "Unable to read/write to IOMMU perf counter.\n");
> -	amd_iommu_pc_present = false;
> -	return;
>   }
>   
>   static ssize_t amd_iommu_show_cap(struct device *dev,
> 

I will test this patch and the revert on my two AMD systems and update
you in a day or two.

thanks,
-- Shuah