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Subject: Re: [PATCH v3 9/9] KVM: Move instrumentation-safe annotations for
 enter/exit to x86 code
To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     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>,
        Sven Schnelle <svens@linux.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>
References: <20210415222106.1643837-1-seanjc@google.com>
 <20210415222106.1643837-10-seanjc@google.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <0c74158d-279a-5afa-0778-822c77ac8dc2@de.ibm.com>
Date:   Wed, 21 Apr 2021 10:09:11 +0200
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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?

So maybe drop this patch until every other arch agrees that there are
no plans to implement this.