On Thu, Oct 06, 2022 at 12:14:17AM +0000, Sean Christopherson wrote: > Date: Thu, 6 Oct 2022 00:14:17 +0000 > From: Sean Christopherson <seanjc@xxxxxxxxxx> > 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@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. > > Uber nit, I would phrase this as saying that local mappings don't disable > page faults and preemption, which is slightly different than stating that they > allow pagefaults/preemption. E.g. if preemption is already disabled. > > > 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 > > Nit, s/kmap_locla_page/kmap_local_page > > For future reference, even better would be to use human language after "introducing" > the functions, e.g. > > The main difference between atomic and local mappings is that local > mappings don't disable page faults or preemption. > > Obviously that doesn't magically prevent typos, but it does make the changelog > easier to read (IMO). > > > 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> > > --- > > No need to send a v3, the above are all the nittiest of nits. Thanks Sean! I'll pay more attention to these next time. Zhao > > Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
