linux-kernel - Re: [PATCH v10 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states

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Date:   Thu, 23 Jul 2020 09:21:16 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v10 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES
 supervisor states

On Thu, 2020-07-23 at 09:10 -0700, Sean Christopherson wrote:
> On Wed, Apr 29, 2020 at 03:07:09PM -0700, Yu-cheng Yu wrote:
> > diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> > index 12c9684d59ba..47f603729543 100644
> > --- a/arch/x86/include/asm/msr-index.h
> > +++ b/arch/x86/include/asm/msr-index.h
> > @@ -885,4 +885,22 @@
> >  #define MSR_VM_IGNNE                    0xc0010115
> >  #define MSR_VM_HSAVE_PA                 0xc0010117
> >  
> > +/* Control-flow Enforcement Technology MSRs */
> > +#define MSR_IA32_U_CET		0x6a0 /* user mode cet setting */
> > +#define MSR_IA32_S_CET		0x6a2 /* kernel mode cet setting */
> > +#define MSR_IA32_PL0_SSP	0x6a4 /* kernel shstk pointer */
> > +#define MSR_IA32_PL1_SSP	0x6a5 /* ring-1 shstk pointer */
> > +#define MSR_IA32_PL2_SSP	0x6a6 /* ring-2 shstk pointer */
> > +#define MSR_IA32_PL3_SSP	0x6a7 /* user shstk pointer */
> > +#define MSR_IA32_INT_SSP_TAB	0x6a8 /* exception shstk table */
> > +
> > +/* MSR_IA32_U_CET and MSR_IA32_S_CET bits */
> > +#define MSR_IA32_CET_SHSTK_EN		0x0000000000000001ULL
> 
> Can we drop the MSR_IA32 prefix for the individual bits?  Mostly to yield
> shorter line lengths, but also because it's more or less redundant info,
> and in some ways unhelpful as it's hard to quickly differentiate between
> "this is an MSR index" and "this is a bit/mask for an MSR".

Agree!

> 
> My vote would also be to use BIT() or BIT_ULL().  The SDM defines the flags
> by their (decimal) bit number.  Manually converting the bits to masks makes
> it difficult to check for correctness.
> 
> E.g.
> 
> #define CET_SHSTK_EN		BIT(0)
> #define CET_WRSS_EN		BIT(1)
> #define CET_ENDBR_EN		BIT(2)
> #define CET_LEG_IW_EN		BIT(3)
> #define CET_NO_TRACK_EN		BIT(4)
> #define CET_WAIT_ENDBR		BIT(5)

I will change them.

> 
> > +#define MSR_IA32_CET_WRSS_EN		0x0000000000000002ULL
> > +#define MSR_IA32_CET_ENDBR_EN		0x0000000000000004ULL
> > +#define MSR_IA32_CET_LEG_IW_EN		0x0000000000000008ULL
> > +#define MSR_IA32_CET_NO_TRACK_EN	0x0000000000000010ULL
> > +#define MSR_IA32_CET_WAIT_ENDBR	0x00000000000000800UL
> > +#define MSR_IA32_CET_BITMAP_MASK	0xfffffffffffff000ULL
> 
> This particular define, the so called BITMAP_MASK, is no longer used in the
> IBT series.  IMO it'd be better off dropping this mask as it's not clear
> from the name that this is really nothing more than a mask for a virtual
> address, e.g. at first glance (for someone without CET knowledge) it looks
> like bits 63:12 hold a bitmap as opposed to holding a pointer to a bitmap.

I will remove this.

Thanks,
Yu-cheng