Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2

[Toshi Kani]

On Fri, 2015-08-07 at 13:25 -0700, Luis R. Rodriguez wrote:
> On Thu, Aug 6, 2015 at 3:58 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> > On Thu, 2015-08-06 at 12:53 -0700, Luis R. Rodriguez wrote:
> > > On Fri, Jun 12, 2015 at 9:58 AM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> > > > On Fri, 2015-06-12 at 08:59 +0100, Jan Beulich wrote:
> > > > > > > > On 12.06.15 at 01:23, <toshi.kani@xxxxxx> wrote:
> > > > > > There are two usages on MTRRs:
> > > > > >  1) MTRR entries set by firmware
> > > > > >  2) MTRR entries set by OS drivers
> > > > > > 
> > > > > > We can obsolete 2), but we have no control over 1).  As UEFI
> > > > > > firmwares also set this up, this usage will continue to stay. 
> > > > > >  So, we should not get rid of the MTRR code that looks up the 
> > > > > > MTRR entries, while we have no need to modify them.
> > > > > > 
> > > > > > Such MTRR entries provide safe guard to /dev/mem, which allows
> > > > > > privileged user to access a range that may require UC mapping 
> > > > > > while the /dev/mem driver blindly maps it with WB.  MTRRs 
> > > > > > converts WB to UC in such a case.
> > > > > 
> > > > > But it wouldn't be impossible to simply read the MTRRs upon boot,
> > > > > store the information, disable MTRRs, and correctly use PAT to
> > > > > achieve the same effect (i.e. the "blindly maps" part of course
> > > > > would need fixing).
> > > > 
> > > > It could be done, but I do not see much benefit of doing it.  One of 
> > > > the reasons platform vendors set MTRRs is so that a system won't hit 
> > > > a machine check when an OS bug leads an access with a wrong cache
> > > > type.
> > > > 
> > > > A machine check is hard to analyze and can be seen as a hardware 
> > > > issue by customers.  Emulating MTRRs with PAT won't protect from 
> > > > such a bug.
> > > 
> > > That's seems like a fair and valid concern. This could only happen if
> > > the OS would have code that would use MTRR, in the case of Linux we'll
> > > soon be able to vet that this cannot happen.
> > 
> > No, there is no OS support necessary to use MTRR.  After firmware sets 
> > it up, CPUs continue to use it without any OS support.  I think the 
> > Linux change you are referring is to obsolete legacy interfaces that 
> > modify the MTRR setup.  I agree that Linux should not modify MTRR.
> 
> Its a bit more than that though. Since you agree that the OS can live
> without MTRR code I was hoping to then see if we can fold out PAT
> Linux code from under the MTRR dependency on Linux and make PAT a
> first class citizen, maybe at least for x86-64. Right now you can only
> get PAT support on Linux if you have MTRR code, but I'd like to see if
> instead we can rip MTRR code out completely under its own Kconfig and
> let it start rotting away.
>
> Code-wise the only issue I saw was that PAT code also relies on
> mtrr_type_lookup(), see pat_x_mtrr_type(), but other than this I found
> no other obvious issues.

We can rip of the MTTR code that modifies the MTRR setup, but not
mtrr_type_lookup().  This function provides necessary checks per documented
in commit 7f0431e3dc89 as follows.

    1) reserve_memtype() tracks an effective memory type in case
       a request type is WB (ex. /dev/mem blindly uses WB). Missing
       to track with its effective type causes a subsequent request
       to map the same range with the effective type to fail.
    
    2) pud_set_huge() and pmd_set_huge() check if a requested range
       has any overlap with MTRRs. Missing to detect an overlap may
       cause a performance penalty or undefined behavior.

mtrr_type_lookup() is still admittedly awkward, but I do not think we have
an immediate issue in PAT code calling it.  I do not think it makes PAT code
a second class citizen. 

> Platform firmware and SMIs seems to be the only other possible issue.
> More on this below.
> 
> > > For those type of OSes...
> > > could it be possible to negotiate or hint to the platform through an
> > > attribute somehow that the OS has such capability to not use MTRR?
> > 
> > The OS can disable MTRR.  However, this can also cause a problem in
> > firmware, which may rely on MTRR.
> 
> Can you describe what type of issues we could expect ? I tend to care
> more about this for 64-bit systems so if 32-bit platforms would be
> more of the ones which could cause an issue would restricting
> disabling MTRR only for 64-bit help?

The SMI handler runs in real-mode and relies on MTRR being effective to
provide right cache types.  It does not matter if it is 64-bit or not.

> > > Then, only if this bit is set, the platform could then avoid such MTRR
> > > settings, and if we have issues you can throw rocks at us.
> > 
> > > And if that's not possible how about a new platform setting that would
> > > need to be set at the platform level to enable disabling this junk?
> > > Then only folks who know what they are doing would enable it, and if
> > > the customer set it, the issue would not be on the platform.
> > 
> > > Could this also be used to prevent SMIs with MTRRs?
> > 
> > ACPI _OSI could be used for firmware to implement some OS-specific 
> > features, but it may be too late for firmware to make major changes and 
> > is generally useless unless OS requirements are described in a spec 
> > backed by logo certification.
> 
> I see.. So there are no guarantees that platform firmware would not
> expect OS MTRR support.
>
> >  SMIs are also used for platform management, such as fan
> > speed control.
> 
> And its conceivable that some devices, or the platform itself, may
> trigger SMIs to have the platform firmware poke with MTRRs?

SMIs are outside of OS control.  SMI handler relies on MTRR being set.  SMI
must be quick, so the handler should not be required to initialize MTRR or
page tables.

> > Is there any issue for Linux to use MTRR set by firmware?
> 
> Even though we don't have the Kconfig option right now to disable MTRR
> cod explicitly I'll note that there are a few other cases that could
> flip Linux to note use MTRR:
> 
>   a) Some BIOSes could let MTRR get disabled
>   b) As of Xen 4.4, the hypervisor disables X86_FEATURE_MTRR which
> disables MTRR on Linux
> 
> If these environments can exist it'd be good to understand possible
> issues that could creep up as a result of the OS not having MTRR
> enabled. If this is a reasonable thing for x86-64 I was hoping we
> could just let users opt-in to a similar build configuration through
> the OS by letting PAT not depend on MTRR.

Case a) and b) do not cause any issue.  They simply lead mtrr_type_lookup()
to return immediately with MTRR_TYPE_INVALID (i.e. MTRR disable), and the
callers handle this value properly.  These cases are only problematic when
the OS tries to modify MTRR.

Thanks,
-Toshi
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