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

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Message-ID: <f620c7d4-6345-4ad0-8a45-c8089e3c34df@citrix.com>
Date:   Sat, 21 Oct 2023 00:55:45 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        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
Cc:     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 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.

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.

~Andrew