linux-kernel - Re: [PATCH v2] KVM: SVM: Replace kmap_atomic() with kmap_local

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Message-ID: <Y4pQK72G37fQIrAq@google.com>
Date:   Fri, 2 Dec 2022 19:21:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zhao Liu <zhao1.liu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2] KVM: SVM: Replace kmap_atomic() with kmap_local_page()

On Wed, Sep 28, 2022, Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@...el.com>
> 
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1].
> 
> The main difference between kmap_atomic() and kmap_local_page() is the
> latter allows pagefaults and preemption.
> 
> There're 2 reasons we can use kmap_local_page() here:
> 1. SEV is 64-bit only and kmap_locla_page() doesn't disable migration in
> this case, but here the function clflush_cache_range() uses CLFLUSHOPT
> instruction to flush, and on x86 CLFLUSHOPT is not CPU-local and flushes
> the page out of the entire cache hierarchy on all CPUs (APM volume 3,
> chapter 3, CLFLUSHOPT). So there's no need to disable preemption to ensure
> CPU-local.
> 2. clflush_cache_range() doesn't need to disable pagefault and the mapping
> is still valid even if sleeps. This is also true for sched out/in when
> preempted.
> 
> In addition, though kmap_local_page() is a thin wrapper around
> page_address() on 64-bit, kmap_local_page() should still be used here in
> preference to page_address() since page_address() isn't suitable to be used
> in a generic function (like sev_clflush_pages()) where the page passed in
> is not easy to determine the source of allocation. Keeping the kmap* API in
> place means it can be used for things other than highmem mappings[2].
> 
> Therefore, sev_clflush_pages() is a function that should use
> kmap_local_page() in place of kmap_atomic().
> 
> Convert the calls of kmap_atomic() / kunmap_atomic() to kmap_local_page() /
> kunmap_local().
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> [2]: https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@intel.com/
> 
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Zhao Liu <zhao1.liu@...el.com>
> ---

Merged to kvm/queue, thanks!

https://lore.kernel.org/all/Y4lHxds8pvBhxXFX@google.com