linux-kernel - Re: [PATCH] KVM: SVM: Replace kmap_atomic() with kmap_local

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Message-ID: <YxTKJ0O3Gd6at5lH@liuzhao-OptiPlex-7080>
Date:   Sun, 4 Sep 2022 23:54:15 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>
Subject: Re: [PATCH] KVM: SVM: Replace kmap_atomic() with kmap_local_page()

On Fri, Sep 02, 2022 at 08:09:29AM -0700, Dave Hansen wrote:
> This changelog is a little on the weak side.  You could literally take
> any arbitrary call-site and file for kmap_atomic() and slap that
> changelog on it.  For instance:
> 
> 	In drivers/target/target_core_sbc.c, the function
> 	sbc_dif_copy_prot() doesn't need to disable pagefaults and
> 	preemption in kmap_atomic(). It can simply use kmap_local_page()
> 	/ kunmap_local() that can instead do the mapping / unmapping
> 	regardless of the context.
> 
> 	With kmap_local_page(), the mapping is per thread, CPU local and
> 	not globally visible. Therefore, sbc_dif_copy_prot() is a
> 	function where the use of kmap_local_page() in place of
> 	kmap_atomic() is correctly suited.
> 
> That's all valid English and there's nothing incorrect in it.  But, it
> doesn't indicate that any actual analysis was performed.  It's utterly
> generic.  It could literally have been generated by a pretty trivial script.
> 
> It would be great to add at least a small, call-site-specific human
> touch to these changelogs.
> 
> In this case, saying something about how global the cache flush is would
> be a great addition.
Thanks Dave! I'll add more detail to the commit message and avoid overly
templated changelogs.

Zhao