linux-kernel - Re: [PATCH 2/4] kvm: svm: Enable UMIP feature on AMD

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Message-ID: <CALCETrX9ztjRvCMFTWrf0WgAv4C9Y9DWcxQ4bBr3ajcAWNF9ZA@mail.gmail.com>
Date:   Fri, 1 Nov 2019 12:09:06 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Moger, Babu" <Babu.Moger@....com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/4] kvm: svm: Enable UMIP feature on AMD

On Fri, Nov 1, 2019 at 11:38 AM Moger, Babu <Babu.Moger@....com> wrote:
>
>
>
> On 11/1/19 1:24 PM, Andy Lutomirski wrote:
> > On Fri, Nov 1, 2019 at 10:33 AM Moger, Babu <Babu.Moger@....com> wrote:
> >>
> >> AMD 2nd generation EPYC processors support UMIP (User-Mode Instruction
> >> Prevention) feature. The UMIP feature prevents the execution of certain
> >> instructions if the Current Privilege Level (CPL) is greater than 0.
> >> If any of these instructions are executed with CPL > 0 and UMIP
> >> is enabled, then kernel reports a #GP exception.
> >>
> >> The idea is taken from articles:
> >> https://lwn.net/Articles/738209/
> >> https://lwn.net/Articles/694385/
> >>
> >> Enable the feature if supported on bare metal and emulate instructions
> >> to return dummy values for certain cases.
> >
> > What are these cases?
>
> It is mentioned in the article https://lwn.net/Articles/738209/
>
> === How does it impact applications?
>
> When enabled, however, UMIP will change the behavior that certain
> applications expect from the operating system. For instance, programs
> running on WineHQ and DOSEMU2 rely on some of these instructions to
> function. Stas Sergeev found that Microsoft Windows 3.1 and dos4gw use the
> instruction SMSW when running in virtual-8086 mode [4]. SGDT and SIDT can
> also be used on virtual-8086 mode.
>

What does that have to do with your series?  Your series is about
enabling UMIP (or emulating UMIP -- your descriptions are quite
unclear) on AMD hardware, and the hypervisor should *not* be emulating
instructions to return dummy values.  The *guest kernel* already knows
how to emulate userspace instructions as needed.

--Andy