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

[Shuah Khan]

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@xxxxxxxxxxxxxxxxxxxxxxx/
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201753
> Cc: Tj (Elloe Linux) <ml.linux@elloe.vision>
> Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
> Cc: Alexander Monakov <amonakov@xxxxxxxxx>
> Cc: David Coe <david.coe@xxxxxxxxxx>
> Cc: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> ---


Tested-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>

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