linux-kernel - Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

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Message-ID: <20231021011859.c2rtc4vl7l2cl4q6@desk>
Date:   Fri, 20 Oct 2023 18:18:59 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
> On 20/10/2023 9:44 pm, Pawan Gupta wrote:
> > +#define EXEC_VERW				\
> > +	__EXEC_VERW(551f);			\
> > +	/* nopl __KERNEL_DS(%rax) */		\
> > +	.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;	\
> > +551:	.word __KERNEL_DS;			\
> 
> Is this actually wise from a perf point of view?
> 
> You're causing a data access to the instruction stream, and not only
> that, the immediate next instruction.  Some parts don't take kindly to
> snoops hitting L1I.

I suspected the same and asked CPU architects, they did not anticipate
reads being interpreted as part of self modifying code. The perf numbers
do not indicate a problem, but they dont speak for all the parts. It
could be an issue with some parts.

> A better option would be to simply have
> 
> .section .text.entry
> .align CACHELINE
> mds_verw_sel:
>     .word __KERNEL_DS
>     int3
> .align CACHELINE
> 
> 
> And then just have EXEC_VERW be
> 
>     verw mds_verw_sel(%rip)
> 
> in the fastpaths.  That keeps the memory operand in .text.entry it works
> on Meltdown-vulnerable CPUs, but creates effectively a data cacheline
> that isn't mixed into anywhere in the frontend, which also gets far
> better locality of reference rather than having it duplicated in 9
> different places.

> Also it avoids playing games with hiding data inside an instruction.
> It's a neat trick, but the neater trick is avoid it whenever possible.

Thanks for the pointers. I think verw in 32-bit mode won't be able to
address the operand outside of 4GB range. Maybe this is fine or could it
be a problem addressing from e.g. KVM module?