linux-kernel - Re: [PATCH v5 2/5] perf: Add SNOOP_PEER flag to perf mem data struct

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Message-ID: <20220422212249.22463-1-alisaidi@amazon.com>
Date:   Fri, 22 Apr 2022 21:22:49 +0000
From:   Ali Saidi <alisaidi@...zon.com>
To:     <kan.liang@...ux.intel.com>
CC:     <Nick.Forrington@....com>, <acme@...nel.org>, <ak@...ux.intel.com>,
        <alexander.shishkin@...ux.intel.com>, <alisaidi@...zon.com>,
        <andrew.kilroy@....com>, <benh@...nel.crashing.org>,
        <german.gomez@....com>, <james.clark@....com>,
        <john.garry@...wei.com>, <jolsa@...nel.org>, <kjain@...ux.ibm.com>,
        <leo.yan@...aro.org>, <lihuafei1@...wei.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-perf-users@...r.kernel.org>,
        <mark.rutland@....com>, <mathieu.poirier@...aro.org>,
        <mingo@...hat.com>, <namhyung@...nel.org>, <peterz@...radead.org>,
        <will@...nel.org>
Subject: Re: [PATCH v5 2/5] perf: Add SNOOP_PEER flag to perf mem data struct


On Fri, 22 Apr 2022 21:43:28, Kan Liang wrote:
> On 4/22/2022 2:49 PM, Ali Saidi wrote:
> > On Wed, 20 Apr 2022 18:43:28, Kan Liang wrote:
> >> On 4/8/2022 3:53 PM, Ali Saidi wrote:
> >>> Add a flag to the perf mem data struct to signal that a request caused a
> >>> cache-to-cache transfer of a line from a peer of the requestor and
> >>> wasn't sourced from a lower cache level.
> >>
> >> It sounds similar to the Forward state. Why can't the
> >> PERF_MEM_SNOOPX_FWD be reused?
> > 
> > Is there a definition of SNOOPX_FWD i can refer to? Happy to use this instead if
> > the semantics align between architectures.
> > 
> 
> + Andi
> 
> As my understanding, the SNOOPX_FWD means the Forward state, which is a 
> non-modified (clean) cache-to-cache copy.
> https://en.wikipedia.org/wiki/MESIF_protocol
  
In this case the semantics are different. We know the line was transferred from
another peer cache, but don't know if it was clean, dirty, or if the receiving core
now has exclusive ownership of it.

Thanks,

Ali