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

[Toshi Kani]

On Fri, 2015-08-07 at 15:23 -0700, Luis R. Rodriguez wrote:
> On Fri, Aug 7, 2015 at 2:56 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> > 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:
 :
> > > > 
> > > > 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.
> 
> OK since we know that if MTRR set up code ends up disabled and would
> return MTRR_TYPE_INVALID what if we just static inline this for the
> no-MTRR Kconfig build option immediately, and only then have the full
> blown implementation for the case where MTRR Kconfig option is
> enabled?

Yes, the MTRR code could be disabled by Kconfig with such inline stubs as
long as the kernel is built specifically for a particular platform with MTRR
disabled, such as Xen guest kernel.

However, since MTRR is a CPU feature enabled on most of the systems, I am
not sure if it makes sense to be configurable with Kconfig, though.

> > > 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.
> 
> I see... since I have no visibility to what goes under the hood, can
> you provide one example use case where an SMI handler would require
> getting a cache type through MTRR ? I realize this can vary, vendor by
> vendor, but any example would do just to satisfy my curiosity.

For fan control, it would need UC access to its registers.

> > > > > 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.
> 
> Right makes sense.
> 
> > > > 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.
> 
> OK if the OS returns MTRR_TYPE_INVALID, for folks who do not want MTRR
> code on their kernel, we should be OK?

Technically OK.  Not sure if we want such a Kconfig option, though.

Thanks,
-Toshi
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