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

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Message-ID: <2ca6c68f-16a7-402f-adb0-327583695d4a@zytor.com>
Date: Tue, 30 Sep 2025 12:59:11 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc: 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 2025-09-30 12:49, 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.
> 

You can, of course, also simply have a conditional jump, at the expense of
making the whole alternative block one byte longer:

  0:   e8 xx xx xx xx          call   asm_xen_pv_wrmsr
  5:   0f 82 xx xx xx xx       jc     wrmsr_failed

	-hpa