From: Zhao Liu <zhao1.liu@xxxxxxxxx> 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@xxxxxxxxx [2]: https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@xxxxxxxxx/ Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx> Suggested-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx> --- 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
