linux-kernel - Re: [PATCH 3/3] KVM/VMX: Use try_cmpxchg64() in posted

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Message-ID: <YAHs6KDoc+O50beV@google.com>
Date:   Fri, 15 Jan 2021 11:28:40 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        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>
Subject: Re: [PATCH 3/3] KVM/VMX: Use try_cmpxchg64() in posted_intr.c

On Tue, Dec 15, 2020, Uros Bizjak wrote:
> Use try_cmpxchg64() instead of cmpxchg64() to reuse flags from
> cmpxchg/cmpxchg8b instruction. For 64 bit targets flags reuse
> avoids a CMP instruction,

It ends up doing way more (in a good way) than eliminate the CMP, at least with
gcc-10.  There's a ripple effect and the compiler ends up generating the loop
in-line, whereas without the "try" version the loop is put out-of-line.

> while for 32 bit targets flags reuse avoids XOR/XOR/OR instruction sequence.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

Reviewed-by: Sean Christopherson <seanjc@...gle.com>