Re: [PATCH v2] UefiCpuPkg\CpuSmm: Save & restore CR2 on-demand paging in SMM

["Kinney, Michael D" <michael.d.kinney@intel.com>]

Laszlo,

Good news is that the PCD being used is a Feature Flag.

[PcdsFeatureFlag]
  ## Indicates if SMM Profile will be enabled.
  #  If enabled, instruction executions in and data accesses to memory outside of SMRAM will be logged.
  #  It could not be enabled at the same time with SMM static page table feature (PcdCpuSmmStaticPageTable).
  #  This PCD is only for validation purpose. It should be set to false in production.<BR><BR>
  #   TRUE  - SMM Profile will be enabled.<BR>
  #   FALSE - SMM Profile will be disabled.<BR>
  # @Prompt Enable SMM Profile.
  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmProfileEnable|FALSE|BOOLEAN|0x32132109

That means a different PcdLib function should be used to look up
the value so it is clear that it is safe at SMM runtime.

    FeaturePcdGet(TokenName)

Best regards,

Mike


> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-
> bounces@lists.01.org] On Behalf Of Laszlo Ersek
> Sent: Thursday, March 7, 2019 9:58 AM
> To: Vanguput, Narendra K
> <narendra.k.vanguput@intel.com>; edk2-
> devel@lists.01.org
> Cc: Yao, Jiewen <jiewen.yao@intel.com>; Dong, Eric
> <eric.dong@intel.com>
> Subject: Re: [edk2] [PATCH v2] UefiCpuPkg\CpuSmm: Save
> & restore CR2 on-demand paging in SMM
> 
> On 03/07/19 12:14, nkvangup wrote:
> > BZ:
> https://bugzilla.tianocore.org/show_bug.cgi?id=1593
> >
> > For every SMI occurrence, save and restore CR2
> register only when SMM
> > on-demand paging support is enabled in 64 bit
> operation mode.
> >
> > Contributed-under: TianoCore Contribution Agreement
> 1.1
> > Signed-off-by: Vanguput Narendra K
> <narendra.k.vanguput@intel.com>
> > Cc: Eric Dong <eric.dong@intel.com>
> > Cc: Ray Ni <ray.ni@intel.com>
> > Cc: Laszlo Ersek <lersek@redhat.com>
> > Cc: Yao Jiewen <jiewen.yao@intel.com>
> > ---
> >  UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c | 20
> ++++++++++++--------
> >  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> (1) There is an open question about the usefulness of
> this patch in
> <https://bugzilla.tianocore.org/show_bug.cgi?id=1593#c1
> >. It should be
> answered in the BZ, or the same description should be
> included in the
> commit message.
> 
> (2) Also, the commit message should refer to the BZ.
> 
> 
> > diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> > index 3b0b3b52ac..5be4a2b020 100644
> > --- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> > +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> > @@ -1111,10 +1111,12 @@ SmiRendezvous (
> >
> >    ASSERT(CpuIndex < mMaxNumberOfCpus);
> >
> > -  //
> > -  // Save Cr2 because Page Fault exception in SMM
> may override its value
> > -  //
> > -  Cr2 = AsmReadCr2 ();
> > +  if ((sizeof (UINTN) == sizeof (UINT64)) &&
> (!PcdGetBool (PcdCpuSmmStaticPageTable))) {
> 
> (3) It doesn't look like a good idea to me to call
> PcdGetBool() in the
> SmiRendezvous() function.
> 
> If the PCD is not fixed-at-build (but dynamic), then
> we'll end up
> calling a PI protocol member from a function that is by
> definition
> executed by multiple processors at the same time.
> 
> "X64/PageTbl.c" already defines the global variable
> "mCpuSmmStaticPageTable", setting it from the PCD on
> the call stack of
> the entry point function of the driver. That is safe --
> we can call PI /
> UEFI protocols in the entry point functions of a
> DXE_SMM_DRIVER.
> 
> Now, the fact that "mCpuSmmStaticPageTable" is only
> defined in the X64
> build (that is, in "X64/PageTbl.c"), is actually quite
> informative. It
> means that, instead of this conditional code in
> "MpService.c", we should
> introduce two new helper functions, "SaveCr2" and
> "RestoreCr2". And we
> should provide separate implementations for IA32 and
> X64. For IA32, the
> function should do nothing. For X64, the function
> should depend on
> "mCpuSmmStaticPageTable", and massage CR2 as necessary.
> 
> However: that *still* depends on whether this change is
> useful. I
> realize the CR2 manipulation may not be overly useful
> on IA32 (we can't
> address >4GB memory, so demand paging for >4GB makes no
> sense), but its
> performance hit should be negligible. Again, back to
> point (1): what is
> the actual issue with the current code?
> 
> Thanks
> Laszlo
> 
> > +    //
> > +    // Save Cr2 because Page Fault exception in SMM
> may override its value
> > +    //
> > +    Cr2 = AsmReadCr2 ();
> > +  }
> >
> >    //
> >    // Perform CPU specific entry hooks
> > @@ -1253,10 +1255,12 @@ SmiRendezvous (
> >
> >  Exit:
> >    SmmCpuFeaturesRendezvousExit (CpuIndex);
> > -  //
> > -  // Restore Cr2
> > -  //
> > -  AsmWriteCr2 (Cr2);
> > +  if ((sizeof (UINTN) == sizeof (UINT64)) &&
> (!PcdGetBool (PcdCpuSmmStaticPageTable))) {
> > +    //
> > +    // Restore Cr2
> > +    //
> > +    AsmWriteCr2 (Cr2);
> > +  }
> >  }
> >
> >  /**
> >
> 
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