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

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

Laszlo,

The information I provided below is incorrect.  The PCD referenced
does support all PCD types as Jiewen noted.

Mike

> -----Original Message-----
> From: Kinney, Michael D
> Sent: Thursday, March 7, 2019 10:10 AM
> To: Laszlo Ersek <lersek@redhat.com>; Vanguput,
> Narendra K <narendra.k.vanguput@intel.com>; edk2-
> devel@lists.01.org; Kinney, Michael D
> <michael.d.kinney@intel.com>
> 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
> 
> 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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