linux-kernel - Re: [PATCH v2 2/3] powerpc/powernv/idle: save-restore DAWR0, DAWRX0 for P10

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Message-ID: <ddf13184-86a3-5b84-6b4f-d901fc884ba5@linux.ibm.com>
Date:   Mon, 20 Jul 2020 10:00:56 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Pratik Rajesh Sampat <psampat@...ux.ibm.com>
Cc:     benh@...nel.crashing.org, ego@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        mikey@...ling.org, mpe@...erman.id.au, paulus@...ba.org,
        pratik.r.sampat@...il.com, svaidy@...ux.ibm.com
Subject: Re: [PATCH v2 2/3] powerpc/powernv/idle: save-restore DAWR0, DAWRX0
 for P10

Hi Nick,

On 7/13/20 11:22 AM, Nicholas Piggin wrote:
> Excerpts from Pratik Rajesh Sampat's message of July 10, 2020 3:22 pm:
>> Additional registers DAWR0, DAWRX0 may be lost on Power 10 for
>> stop levels < 4.
>> Therefore save the values of these SPRs before entering a  "stop"
>> state and restore their values on wakeup.
> 
> Hmm, where do you get this from? Documentation I see says DAWR is lost
> on POWER9 but not P10.
> 
> Does idle thread even need to save DAWR, or does it get switched when
> going to a thread that has a watchpoint set?

I don't know how idle states works internally but IIUC, we need to save/restore
DAWRs. This is needed when user creates per-cpu watchpoint event.

Ravi