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Date:   Fri, 29 Apr 2022 10:40:39 +0530
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Subject: Re: [PATCH v4 6/7] perf/x86/amd/core: Add PerfMonV2 overflow handling
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To:     Like Xu <like.xu.linux@gmail.com>
Cc:     peterz@infradead.org, bp@alien8.de, dave.hansen@linux.intel.com,
        acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, namhyung@kernel.org,
        jolsa@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        pbonzini@redhat.com, jmattson@google.com, eranian@google.com,
        puwen@hygon.cn, ananth.narayan@amd.com, ravi.bangoria@amd.com,
        santosh.shukla@amd.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-perf-users@vger.kernel.org, x86@kernel.org
References: <cover.1651058600.git.sandipan.das@amd.com>
 <6bb3a6de79afbdb1ebc1b804fb8c2002c00cbaee.1651058600.git.sandipan.das@amd.com>
 <5c227ef8-72d2-9254-f061-128974a6dc7d@gmail.com>
From:   Sandipan Das <sandipan.das@amd.com>
In-Reply-To: <5c227ef8-72d2-9254-f061-128974a6dc7d@gmail.com>
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On 4/28/2022 7:10 PM, Like Xu wrote:
> On 27/4/2022 7:31 pm, Sandipan Das wrote:
>> +static inline void amd_pmu_ack_global_status(u64 status)
>> +{
>> +    /*
>> +     * PerfCntrGlobalStatus is read-only but an overflow acknowledgment
> 
> If wrmsrl PerfCntrGlobalStatus, remain silent or report #GP ?
> 

This is a read-only MSR and writes are silently ignored.

>> +     * mechanism exists; writing 1 to a bit in PerfCntrGlobalStatusClr
>> +     * clears the same bit in PerfCntrGlobalStatus
>> +     */
>> +
>> +    /* Only allow modifications to PerfCntrGlobalStatus.PerfCntrOvfl */
>> +    status &= amd_pmu_global_cntr_mask;
>> +    wrmsrl(MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR, status);
> 
> If rdmsrl PerfCntrGlobalStatusClr, does it return 0 or the value of PerfCntrGlobalStatus ?
> 

This is a write-only MSR and reads are undefined.

The "Field Access Type" section from the AMD Processor Programming
Reference (PPR) has these details.


- Sandipan