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SubjectRe: [PATCH 2/2] iommu/amd: Remove performance counter pre-initialization test
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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.
>
> 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.
>
> 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>
> ---


Tested-by: Shuah Khan <skhan@linuxfoundation.org>

thanks,
-- Shuah

Results with this patch on AMD Ryzen 5 PRO 2400GE w/ Radeon Vega
Graphics

sudo ./perf stat -e 'amd_iommu_0/cmd_processed/,
amd_iommu_0/cmd_processed_inv/, amd_iommu_0/ign_rd_wr_mmio_1ff8h/,
amd_iommu_0/int_dte_hit/, amd_iommu_0/int_dte_mis/,
amd_iommu_0/mem_dte_hit/, amd_iommu_0/mem_dte_mis/,
amd_iommu_0/mem_iommu_tlb_pde_hit/, amd_iommu_0/mem_iommu_tlb_pde_mis/,
amd_iommu_0/mem_iommu_tlb_pte_hit/, amd_iommu_0/mem_iommu_tlb_pte_mis/,
amd_iommu_0/mem_pass_excl/, amd_iommu_0/mem_pass_pretrans/,
amd_iommu_0/mem_pass_untrans/, amd_iommu_0/mem_target_abort/,
amd_iommu_0/mem_trans_total/, amd_iommu_0/page_tbl_read_gst/,
amd_iommu_0/page_tbl_read_nst/, amd_iommu_0/page_tbl_read_tot/,
amd_iommu_0/smi_blk/, amd_iommu_0/smi_recv/, amd_iommu_0/tlb_inv/,
amd_iommu_0/vapic_int_guest/, amd_iommu_0/vapic_int_non_guest/' sleep 10

Performance counter stats for 'system wide':

156 amd_iommu_0/cmd_processed/
(33.30%)
80 amd_iommu_0/cmd_processed_inv/
(33.38%)
0 amd_iommu_0/ign_rd_wr_mmio_1ff8h/
(33.40%)
0 amd_iommu_0/int_dte_hit/
(33.43%)
325 amd_iommu_0/int_dte_mis/
(33.44%)
1,951 amd_iommu_0/mem_dte_hit/
(33.45%)
7,589 amd_iommu_0/mem_dte_mis/
(33.49%)
325 amd_iommu_0/mem_iommu_tlb_pde_hit/
(33.45%)
2,460 amd_iommu_0/mem_iommu_tlb_pde_mis/
(33.41%)
2,510 amd_iommu_0/mem_iommu_tlb_pte_hit/
(33.38%)
5,526 amd_iommu_0/mem_iommu_tlb_pte_mis/
(33.33%)
0 amd_iommu_0/mem_pass_excl/
(33.29%)
0 amd_iommu_0/mem_pass_pretrans/
(33.28%)
1,556 amd_iommu_0/mem_pass_untrans/
(33.27%)
0 amd_iommu_0/mem_target_abort/
(33.26%)
3,112 amd_iommu_0/mem_trans_total/
(33.29%)
0 amd_iommu_0/page_tbl_read_gst/
(33.29%)
1,813 amd_iommu_0/page_tbl_read_nst/
(33.25%)
2,242 amd_iommu_0/page_tbl_read_tot/
(33.27%)
0 amd_iommu_0/smi_blk/
(33.29%)
0 amd_iommu_0/smi_recv/
(33.28%)
0 amd_iommu_0/tlb_inv/
(33.28%)
0 amd_iommu_0/vapic_int_guest/
(33.25%)
0 amd_iommu_0/vapic_int_non_guest/
(33.26%)

10.003200316 seconds time elapsed

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