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

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



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.