linux-kernel - Re: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page transition hypervisor callback

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Message-ID: <20240115100011.yvecjjezrkcptnle@box.shutemov.name>
Date: Mon, 15 Jan 2024 13:00:11 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"ardb@...nel.org" <ardb@...nel.org>,
	"Lutomirski, Andy" <luto@...nel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>,
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Cui, Dexuan" <decui@...rosoft.com>,
	"urezki@...il.com" <urezki@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
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	"wei.liu@...nel.org" <wei.liu@...nel.org>,
	"Rodel, Jorg" <jroedel@...e.de>,
	"sathyanarayanan.kuppuswamy@...ux.intel.com" <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	"lstoakes@...il.com" <lstoakes@...il.com>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 1/3] x86/hyperv: Use slow_virt_to_phys() in page
 transition hypervisor callback

On Fri, Jan 12, 2024 at 07:24:35PM +0000, Michael Kelley wrote:
> From: Edgecombe, Rick P <rick.p.edgecombe@...el.com> 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?

Yes, sounds sensible.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov