Re: [RFC] x86: mtrr: Constrain WB MTRR to max phys mem prior to cleanup

[Zhenzhong Duan]

On 2012-09-29 01:37, Peter Hurley wrote:
> On Sun, 2012-09-09 at 23:54 -0400, zhenzhong.duan wrote:
> > On 2012-09-08 02:40, H. Peter Anvin wrote:
> > > On 09/07/2012 10:44 AM, Peter Hurley wrote:
> > >
> > > > diff --git a/arch/x86/kernel/cpu/mtrr/cleanup.c
> > > > b/arch/x86/kernel/cpu/mtrr/cleanup.c
> > > I really don't like it as it introduces yet another user of max_pfn,
> > > which should be going away.  Furthermore, the better question is what
> > > remaining needs there are for MTRR cleanup; historically the reason
> > > was that it prevented the display from being mapped WC via MTRR due to
> > > the MTRR conflict resolution rules favoring UC.
> > For a large memory system, mtrr_cleanup offten fail in most case. Even
> > if it succeed, it often occupy all of MTRR entrys.
> > How was display mapped as WC in above case?
> Without this patch, mtrr_cleanup could not optimize. The original MTRR
> setup from BIOS remained, which left the display as UC (and a lot of log
> spew).
Hi,
I am confused here.
Does HPA means mtrr_cleanup's purpose is to occupy all mtrr entrys and 
prevent display setting a WC entry in it?
As page level will do that in current code? If it is, then mtrr_cleanup 
could be removed now.
> > Why did bios give a lot of space then real mem, for hotplug?
> I assume the reason was for hotplug.
>
> An interesting side note: more recent revisions of this BIOS (rev. A11)
> report one less variable MTRR (so, IA32_MTRRCAP is writable?)
From manual, it's readonly, writing it will lead to #GP.
> > > However, the right way to fix that is to use the PAT interfaces, which
> > > doesn't have this drawback -- then MTRR cleanup becomes entirely
> > > superfluous and the problem goes away.
> > Do you mean disable MTRR totally here?
> Well, since PAT entries marked WC override all MTRR settings, whatever
> the BIOS set the variable MTRRs to becomes irrelevant, so not disabled
> but rather ignored.
Oh, I see, WC in page level take precedence. Is the fix already in upstream?
thanks
zduan
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