linux-kernel - Re: [PATCH v3 9/9] KVM: Move instrumentation-safe annotations for enter/exit to x86 code

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Message-ID: <yt9d4kfypeov.fsf@linux.ibm.com>
Date:   Thu, 22 Apr 2021 16:38:24 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Michael Tokarev <mjt@....msk.ru>,
        Heiko Carstens <heiko.carstens@...ibm.com>, gor@...ux.ibm.com
Subject: Re: [PATCH v3 9/9] KVM: Move instrumentation-safe annotations for
 enter/exit to x86 code

Christian Borntraeger <borntraeger@...ibm.com> writes:

> On 16.04.21 00:21, Sean Christopherson wrote:
>> Drop the instrumentation_{begin,end}() annonations from the common KVM
>> guest enter/exit helpers, and massage the x86 code as needed to preserve
>> the necessary annotations.  x86 is the only architecture whose transition
>> flow is tagged as noinstr, and more specifically, it is the only
>> architecture for which instrumentation_{begin,end}() can be non-empty.
>> No other architecture supports CONFIG_STACK_VALIDATION=y, and s390
>> is the
>> only other architecture that support CONFIG_DEBUG_ENTRY=y.  For
>> instrumentation annontations to be meaningful, both aformentioned configs
>> must be enabled.
>> Letting x86 deal with the annotations avoids unnecessary nops by
>> squashing back-to-back instrumention-safe sequences.
>
> We have considered implementing objtool for s390. Not sure where we
> stand and if we will do this or not. Sven/Heiko?

We are planning to support objtool on s390. Vasily is working on it -
maybe he has some thoughts about this.