RE: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor callback

[Michael Kelley]

From: Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> Sent: Friday, January 12, 2024 9:17 AM
> 
> On Fri, 2024-01-12 at 15:07 +0000, Michael Kelley wrote:
> > The comment is Kirill Shutemov's suggestion based on comments in
> > an earlier version of the patch series.  See [1].   The intent is to
> > prevent
> > some later revision to slow_virt_to_phys() from adding a check for
> > the
> > present bit and breaking the CoCo VM hypervisor callback.  Yes, the
> > comment could get stale, but I'm not sure how else to call out the
> > implicit dependency.  The idea of creating a private version of
> > slow_virt_to_phys() for use only in the CoCo VM hypervisor callback
> > is also discussed in the thread, but that seems worse overall.
> 
> Well, it's not a huge deal, but I would have just put a comment at the
> caller like:
> 
> /*
>  * Use slow_virt_to_phys() instead of vmalloc_to_page(), because it
>  * returns the PFN even for NP PTEs.
>  */

Yes, that comment is added in this patch.

> 
> If someone is changing slow_virt_to_phys() they should check the
> callers to make sure they won't break anything. They can see the
> comment then and have an easy time.
> 
> An optional comment at slow_virt_to_phys() could explain how the
> function works in regards to the present bit, but not include details
> about CoCoO VM page transition's usage of the present bit. The proposed
> comment text looks like something more appropriate for a commit log.

Kirill -- you originally asked for a comment in slow_virt_to_phys(). [1]
Are you OK with Rick's proposal?

Michael

[1] https://lore.kernel.org/lkml/20230828221333.5blshosyqafbgwlc@xxxxxxxxxxxxxxxxx/