Re: [PATCH v2] KVM: SVM: Replace kmap_atomic() with kmap_local_page()

[Zhao Liu]

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>