linux-kernel - Re: [PATCH] iommu/amd: Fix event counter availability check

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Date:   Mon, 1 Jun 2020 14:37:46 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Alexander Monakov <amonakov@...ras.ru>,
        linux-kernel@...r.kernel.org
Cc:     Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] iommu/amd: Fix event counter availability check

Hi Alexander,

On 5/30/20 3:07 AM, Alexander Monakov wrote:
> The driver performs an extra check if the IOMMU's capabilities advertise
> presence of performance counters: it verifies that counters are writable
> by writing a hard-coded value to a counter and testing that reading that
> counter gives back the same value.
> 
> Unfortunately it does so quite early, even before pci_enable_device is
> called for the IOMMU, i.e. when accessing its MMIO space is not
> guaranteed to work. On Ryzen 4500U CPU, this actually breaks the test:
> the driver assumes the counters are not writable, and disables the
> functionality.
> 
> Moving init_iommu_perf_ctr just after iommu_flush_all_caches resolves
> the issue. This is the earliest point in amd_iommu_init_pci where the
> call succeeds on my laptop.

According to your description, it should just need to be anywhere after the pci_enable_device() is called for the IOMMU 
device, isn't it? So, on your system, what if we just move the init_iommu_perf_ctr() here:

diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
index 5b81fd16f5fa..17b9ac9491e0 100644
--- a/drivers/iommu/amd_iommu_init.c
+++ b/drivers/iommu/amd_iommu_init.c
@@ -1875,6 +1875,7 @@ static int __init amd_iommu_init_pci(void)
                 ret = iommu_init_pci(iommu);
                 if (ret)
                         break;
+               init_iommu_perf_ctr(iommu);
         }

         /*
--
2.17.1


Does this works?

Thanks,
Suravee