linux-kernel - Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

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Date:   Fri, 15 Sep 2023 01:33:14 +0100
From:   andrew.cooper3@...rix.com
To:     Thomas Gleixner <tglx@...utronix.de>, Xin Li <xin3.li@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, luto@...nel.org,
        pbonzini@...hat.com, seanjc@...gle.com, peterz@...radead.org,
        jgross@...e.com, ravi.v.shankar@...el.com, mhiramat@...nel.org,
        jiangshanlai@...il.com
Subject: Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

On 15/09/2023 1:12 am, Thomas Gleixner wrote:
> On Fri, Sep 15 2023 at 00:46, andrew wrote:
>> On 15/09/2023 12:00 am, Thomas Gleixner wrote:
>>> So no. I'm fundamentally disagreeing with your recommendation. The way
>>> forward is:
>>>
>>>   1) Provide the native variant for wrmsrns(), i.e. rename the proposed
>>>      wrmsrns() to native_wrmsr_ns() and have the X86_FEATURE_WRMSRNS
>>>      safety net as you pointed out.
>>>
>>>      That function can be used in code which is guaranteed to be not
>>>      affected by the PV_XXL madness.
>>>
>>>   2) Come up with a sensible solution for the PV_XXL horrorshow
>>>
>>>   3) Implement a sane general variant of wrmsr_ns() which handles
>>>      both X86_FEATURE_WRMSRNS and X86_MISFEATURE_PV_XXL
>>>
>>>   4) Convert other code which benefits from the non-serializing variant
>>>      to wrmsr_ns()
>> Well - point 1 is mostly work that needs reverting as part of completing
>> point 3, and point 2 clearly needs doing irrespective of anything else.
> No. #1 has a value on its own independent of the general variant in #3.
>
>>>      That function can be used in code which is guaranteed to be not
>>>      affected by the PV_XXL madness.
> That makes a lot of sense because it's guaranteed to generate better
> code than whatever we come up with for the PV_XXL nonsense.

It's an assumption about what "definitely won't" be paravirt in the future.

XenPV stack handling is almost-FRED-like and has been for the better
part of two decades.

You frequently complain that there's too much black magic holding XenPV
together.  A paravirt-FRED will reduce the differences vs native
substantially.

~Andrew