linux-kernel - Re: [PATCH V8 14/44] x86/pkeys: Introduce pks_write

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Message-ID: <20220129001653.GM785175@iweiny-DESK2.sc.intel.com>
Date:   Fri, 28 Jan 2022 16:16:53 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 14/44] x86/pkeys: Introduce pks_write_pkrs()

On Fri, Jan 28, 2022 at 04:12:06PM -0800, Dave Hansen wrote:
> On 1/27/22 09:54, ira.weiny@...el.com wrote:
> > Writing to MSR's is inefficient.  Even though the underlying
> > WRMSR(MSR_IA32_PKRS) is not serializing (see below), writing to the MSR
> > unnecessarily should be avoided.  This is especially true when the value
> > of the PKS protections is unlikely to change from the default often.
> 
> This probably needs some context.
> 
> The most important pks_write_pkrs() user is in the scheduler, right?

This is also used during exceptions, twice.  Those are probably more important.

> 
> So, this is really about optimizing that scheduler code for the common
> case where, even when changing threads, the PKRS value does not change.
> 
> Can you explain a bit why you expect that to be the common case?

Yes.

Ira