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

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Message-ID: <5f6871b5-243c-457f-82f7-47246b6ef6ea@suse.com>
Date: Mon, 29 Sep 2025 13:07:41 +0200
From: Juergen 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 26.08.25 12:39, Jürgen Groß wrote:
> On 25.08.25 03:54, Xin Li wrote:
>> On 6/11/2025 5:58 AM, Juergen Gross wrote:
>>> I'm just doing a V2 of my series, but this time including the additional
>>> support of the non-serializing and immediate forms. Lets see how this will
>>> look like. I will drop using the EAX_EDX_* macros, but due to the reason
>>> mentioned above I won't switch to your variant completely.
>>
>> Hi Juergen,
>>
>> Do you have any update on this?
> 
> I've been very busy with other stuff (downstream, security, ...).
> 
> In between I've been working on the series. I hope to post it some time in
> September.

I have been working on this the last week.

Turns out things are a little bit complicated for adding them into the
paravirt framework, especially regarding the exception fixups.

I first thought that Peter's patch "x86/extable: Implement EX_TYPE_FUNC_REWIND"
would help, but I'm seeing problems with his approach in case of shadow stack
being enabled. This case would at least needed to be handled in his patch, as
otherwise shadow stack and normal stack could get out of sync.

For this reason your patch series won't work easily, too.

OTOH using your basic idea it seems to be possible to solve the fixup problem
without needing Peter's patch. I'm working on that approach now.

Juergen

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