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From:   Sven Schnelle <svens@linux.ibm.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Michael Tokarev <mjt@tls.msk.ru>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, gor@linux.ibm.com
Subject: Re: [PATCH v3 9/9] KVM: Move instrumentation-safe annotations for
 enter/exit to x86 code
References: <20210415222106.1643837-1-seanjc@google.com>
        <20210415222106.1643837-10-seanjc@google.com>
        <0c74158d-279a-5afa-0778-822c77ac8dc2@de.ibm.com>
Date:   Thu, 22 Apr 2021 16:38:24 +0200
In-Reply-To: <0c74158d-279a-5afa-0778-822c77ac8dc2@de.ibm.com> (Christian
        Borntraeger's message of "Wed, 21 Apr 2021 10:09:11 +0200")
Message-ID: <yt9d4kfypeov.fsf@linux.ibm.com>
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Christian Borntraeger <borntraeger@de.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.