linux-kernel - Re: [PATCH v2 11/12] x86/paravirt: Don't use pv

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Message-ID: <20251001064543.GM4067720@noisy.programming.kicks-ass.net>
Date: Wed, 1 Oct 2025 08:45:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Jürgen Groß <jgross@...e.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	virtualization@...ts.linux.dev, xin@...or.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.makhalov@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v2 11/12] x86/paravirt: Don't use pv_ops vector for MSR
 access functions

On Tue, Sep 30, 2025 at 12:49:21PM -0700, H. Peter Anvin wrote:

> /* Xen code, stub sets CF = 1 on failure */
> 
>    0:   e8 xx xx xx xx          call   asm_xen_pv_wrmsr
>    5:   73 03                   jnc    0xa
>    7:   0f 0b                   ud2
>    9:   90                      nop
>    a:
> 
> The trap point even ends up in the same place! UD2 can be any 1-, 2-, or
> 3-byte trapping instruction.

Please don't rely on flags to be retained by RET. The various
mitigations have trouble with that.