linux-kernel - Re: [PATCH 5/6] x86/paravirt: Switch MSR access pv

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Message-ID: <bd565df5-b87c-42b1-a717-9ed1267df0c2@suse.com>
Date: Fri, 13 Jun 2025 10:01:09 +0200
From: Jürgen Groß <jgross@...e.com>
To: Xin Li <xin@...or.com>, linux-kernel@...r.kernel.org, x86@...nel.org,
 virtualization@...ts.linux.dev
Cc: Ajay Kaher <ajay.kaher@...adcom.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 xen-devel@...ts.xenproject.org, Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 5/6] x86/paravirt: Switch MSR access pv_ops functions to
 instruction interfaces

On 13.06.25 09:31, Xin Li wrote:
> On 6/11/2025 5:58 AM, Juergen Gross wrote:
>>> Here is a patch I cooked.  I added an ALTERNATIVE() hack because the new 
>>> instructions can't be more than 6 bytes long.  But with the patch you
>>> just sent, it shouldn't be needed.
>>
>> I have meanwhile dropped the patch copying the original indirect call.
>>
>> Reason is that I'm seeing a potential risk with current alternative
>> patching when using ALTERNATIVE_[23](): depending on the tested features
>> it might happen that an instruction sequence not suitable for the current
>> runtime environment is patched in as an intermediate step. In case there
>> is an interrupt happening just then AND the handling of the interrupt is
>> using the patch site, this could result in crashes or undefined behavior.
> 
> Oh, I had assumed that Linux disables interrupts during the patching
> process. Just out of curiosity, why are interrupts allowed in this case?

Interrupts are disabled within text_poke_early() while patching a single
instance.

I guess keeping interrupts disabled during the complete apply_alternatives()
handling would potentially result in a too long period without handling any
interrupts.

Juergen

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