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

[Toshi Kani]

On Fri, 2015-08-07 at 16:26 -0700, Luis R. Rodriguez wrote:
> On Fri, Aug 7, 2015 at 4:08 PM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> > 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:
 :
> > > > > 
> > > > > 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
> 
> OK thanks.
> 
> > as
> > long as the kernel is built specifically for a particular platform with 
> > MTRR disabled, such as Xen guest kernel.
> 
> Sure.
> 
> > 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.
> 
> To me this is about making PAT a first class citizen in code though
> and validating through Kconfig the option then to opt-out of MTRR from
> OS code. Perhaps we can recommend to enable it but having the options
> to split out PAT from MTRR is what I was aiming for.

Since we have CONFIG_MTRR already, we do not need to argue over this option.
:-)  It makes sense since when MTRR code was introduced, there were CPUs
without this capability...

> > > > > 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.
> 
> OK thanks! To follow up with the example, since the platform firmware
> would have set up the MTRRs anyway, the SMI should still work, even if
> the OS didn't do anything, right?

Yes, MTRR works without the OS code.  However, mtrr_type_lookup() is
necessary to make sure that OS mapping requests are aligned with with the
MTRR setup.


> > > > > > 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.
> 
> Its more of me wanting to get PAT out from under MTRR. Does that make 
> sense?

It makes sense if you need to make the kernel size a bit smaller, and you
build kernels specific to Xen guests.  Leaving the MTRR code enabled on Xen
guests does not cause you any issue, though.

Thanks,
-Toshi
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