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Date:   Sat, 12 Feb 2022 00:12:09 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kcc@...gle.com" <kcc@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "pavel@....cz" <pavel@....cz>, "arnd@...db.de" <arnd@...db.de>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "john.allen@....com" <john.allen@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>
CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support

On Fri, 2022-02-11 at 15:37 -0800, Dave Hansen wrote:
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > Add the user shadow stack MSRs to the xsave helpers, so they can be
> > used
> > to implement the functionality.
> 
> Do these MSRs ever affect kernel-mode operation?
> 
> If so, we might need to switch them more aggressively at context-
> switch
> time like PKRU.
> 
> If not, they can continue to be context-switched with the PASID state
> which does not affect kernel-mode operation.
> 
> Either way, it would be nice to have some changelog material to that
> effect.

The only special shadow stack thing the kernel does is WRUSS, which per
the SDM only needs the CR4 bit set to work (unlike WRSS). So I think
the lazy restore is ok.

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