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

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Message-Id: <20220928092748.463631-1-zhao1.liu@linux.intel.com>
Date:   Wed, 28 Sep 2022 17:27:48 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     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>,
        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
Cc:     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: [PATCH v2] KVM: SVM: Replace kmap_atomic() with kmap_local_page()

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>
---
Suggested by credits:
  Dave: Referred to his explanation about cache flush and usage of
        page_address().
  Ira: Referred to his task document, review comments and explanation about
       cache flush.
  Fabio: Referred to his boiler plate commit message.
---
Changes since v1:
  * Add the explanation of global cache flush for sev_clflush_pages() in commit
    message.
  * Add the explanation why not use page_address() directly.
---
 arch/x86/kvm/svm/sev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 28064060413a..12747c7bda4e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -465,9 +465,9 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
 		return;

 	for (i = 0; i < npages; i++) {
-		page_virtual = kmap_atomic(pages[i]);
+		page_virtual = kmap_local_page(pages[i]);
 		clflush_cache_range(page_virtual, PAGE_SIZE);
-		kunmap_atomic(page_virtual);
+		kunmap_local(page_virtual);
 		cond_resched();
 	}
 }
-- 
2.34.1