linux-kernel - Re: [PATCH 2/2] iommu/amd: Remove performance counter pre-initialization test

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Date:   Fri, 9 Apr 2021 21:11:30 +0100
From:   David Coe <david.coe@...e.co.uk>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Cc:     joro@...tes.org, will@...nel.org, jsnitsel@...hat.com,
        pmenzel@...gen.mpg.de, Jon.Grimm@....com,
        Tj <ml.linux@...oe.vision>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Alexander Monakov <amonakov@...ras.ru>,
        Alex Hung <1917203@...s.launchpad.net>
Subject: Re: [PATCH 2/2] iommu/amd: Remove performance counter
 pre-initialization test

On 09/04/2021 09:58, 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.
> 
> Unfortunatly, there is no documentation of since which generation
> 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
> that the buggy platforms are less likely to be in used, and it should
> be relatively safe to remove this legacy logic.

Thanks for explaining the 'context', Suravee.

> 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@...oe.vision>
> Cc: Shuah Khan <skhan@...uxfoundation.org>
> Cc: Alexander Monakov <amonakov@...ras.ru>
> Cc: David Coe <david.coe@...e.co.uk>
> Cc: Paul Menzel <pmenzel@...gen.mpg.de>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....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'll test your revert + update IOMMU patch on my Ryzen 2400G and 4700U 
most likely over the weekend. Very interesting!

Please be aware that your original IOMMU patch has already reached the 
imminent release of Ubuntu 21.04 (Hirsute). I've taken the liberty of 
adding Alex Hung (lead kernel developer at Ubuntu) to the circulation list.