linux-kernel - Re: [PATCH 4/4] x86/xen: drop USERGS

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Message-ID: <194ffa2c-cfc6-29b2-5ee4-3d02581b8e28@suse.com>
Date:   Tue, 17 Nov 2020 09:19:17 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     xen-devel <xen-devel@...ts.xenproject.org>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH 4/4] x86/xen: drop USERGS_SYSRET64 paravirt call

On 16.11.20 17:28, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 7:23 AM Juergen Gross <jgross@...e.com> wrote:
>>
>> USERGS_SYSRET64 is used to return from a syscall via sysret, but
>> a Xen PV guest will nevertheless use the iret hypercall, as there
>> is no sysret PV hypercall defined.
>>
>> So instead of testing all the prerequisites for doing a sysret and
>> then mangling the stack for Xen PV again for doing an iret just use
>> the iret exit from the beginning.
>>
>> This can easily be done via an ALTERNATIVE like it is done for the
>> sysenter compat case already.
>>
>> While at it remove to stale sysret32 remnants.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
> 
> Acked-by: Andy Lutomirski <luto@...nel.org>
> 
> FWIW, you've lost the VGCF_in_syscall optimization.  Let me see if I
> can give it back to you better.

Ah, right.

Nevertheless a simple kernel build is about 0.5% faster with this
patch.


Juergen

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