linux-kernel - Re: [RFC v2 01/32] x86/paravirt: Introduce CONFIG_PARAVIRT

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Date:   Wed, 12 May 2021 06:24:17 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 01/32] x86/paravirt: Introduce CONFIG_PARAVIRT_XL


On 5/12/2021 6:18 AM, Peter Zijlstra wrote:
> On Mon, May 10, 2021 at 05:56:05PM +0200, Juergen Gross wrote:
>
>> No. We have PARAVIRT_XXL for Xen PV guests, and we have PARAVIRT for
>> other hypervisor's guests, supporting basically the TLB flush operations
>> and time related operations only. Adding the halt related operations to
>> PARAVIRT wouldn't break anything.
> Also, I don't think anything modern should actually ever hit any of the
> HLT instructions, most everything should end up at an MWAIT.
>
> Still, do we wants to give arch_safe_halt() and halt() the
> PVOP_ALT_VCALL0() treatment?

 From performance reasons it's pointless to patch. HLT (and MWAIT) are 
so slow anyways that using patching or an indirect pointer is completely 
in the noise. So I would use whatever is cleanest in the code.

-Andi