linux-kernel - Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

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Date:   Thu, 23 Jul 2020 09:52:04 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Weiny Ira <ira.weiny@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across
 exceptions

Hi, Dave,

On Thu, Jul 23, 2020 at 09:23:13AM -0700, Dave Hansen wrote:
> On 7/23/20 9:18 AM, Fenghua Yu wrote:
> > The PKRS MSR has been preserved in thread_info during kernel entry. We
> > don't need to preserve it in another place (i.e. idtentry_state).
> 
> I'm missing how the PKRS MSR gets preserved in thread_info.  Could you
> explain the mechanism by which this happens and point to the code
> implementing it, please?

[Sorry, my mistake: I mean "thread_struct" instead of "thread_info".
Hopefully the typo doesn't change the essential part in my last email.]

The "saved_pkrs" is defined in thread_struct and context switched in
patch 04/17:
https://lore.kernel.org/lkml/20200717072056.73134-5-ira.weiny@intel.com/

Because there is no XSAVE support the PKRS MSR, we preserve it in
"saved_pkrs" in thread_struct. It's initialized as 0 (init state, no
protection key) in fork() or exec(). It's updated to a right protection
value when a driver calls the updating API. The PKRS MSR is context
switched by "saved_pkrs" when switching to a task (unless optimized if the
cached MSR is the same as the saved one).

Thanks.

-Fenghua