Re: [Xen-devel] [PATCH v8 1/5] x86/p2m: Allow p2m_get_page_from_gfn to return shared entries

From: Tamas K Lengyel <tamas.k.lengyel@gmail.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Tamas K Lengyel" <tamas.lengyel@intel.com>,
	"Wei Liu" <wl@xen.org>,
	"George Dunlap" <george.dunlap@eu.citrix.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v8 1/5] x86/p2m: Allow p2m_get_page_from_gfn to return shared entries
Date: Tue, 11 Feb 2020 03:29:17 -0700	[thread overview]
Message-ID: <CABfawhn6Z3BO6JTEDzcR9TUYaOLQHv1KLOHv1LHXy-5Bw5QSog@mail.gmail.com> (raw)
In-Reply-To: <f620590a-147b-eeb8-707c-72cac10753bd@suse.com>

On Tue, Feb 11, 2020 at 2:17 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 10.02.2020 20:21, Tamas K Lengyel wrote:
> > The owner domain of shared pages is dom_cow, use that for get_page
> > otherwise the function fails to return the correct page under some
> > situations. The check if dom_cow should be used was only performed in
> > a subset of use-cases. Fixing the error and simplifying the existing check
> > since we can't have any shared entries with dom_cow being NULL.
> >
> > Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
>
> I find it quite disappointing that the blank lines requested to be
> added ...
>
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -574,11 +574,12 @@ struct page_info *p2m_get_page_from_gfn(
> >                  if ( fdom == NULL )
> >                      page = NULL;
> >              }
> > -            else if ( !get_page(page, p2m->domain) &&
> > -                      /* Page could be shared */
> > -                      (!dom_cow || !p2m_is_shared(*t) ||
> > -                       !get_page(page, dom_cow)) )
> > -                page = NULL;
> > +            else
> > +            {
> > +                struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
> > +                if ( !get_page(page, d) )
>
> .. above here and ...
>
> > @@ -594,8 +595,9 @@ struct page_info *p2m_get_page_from_gfn(
> >      mfn = get_gfn_type_access(p2m, gfn_x(gfn), t, a, q, NULL);
> >      if ( p2m_is_ram(*t) && mfn_valid(mfn) )
> >      {
> > +        struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
> >          page = mfn_to_page(mfn);
>
> ... above here still haven't appeared. No matter that it's easy to
> do so while committing, when you send a new version you should
> really address such remarks yourself, I think.

Noted. I haven't addressed it since it appeared to me that this patch
has been ready to go in for like 3 revisions already as-is given the
blank-lines were non-blockers. By the time I get around rolling a new
one I simply forget nuisance style issues like this. I know we have
been having the discussion about having automated style-checks and
style-formatting added to Xen, this just further highlights to me the
need for it as we are wasting time and energy on stuff like this for
no real reason.

Tamas

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