You're right that there's no such need. I just saw that this driver is loaded before EndOfDxe but missed the fact that it's actually started after that. So BIT7 of PcdNullPointerDetectionPropertyMask is enough.
And thanks a lot for other feedbacks in another emails, especially for the catching of potential attributes overridden issue, which also exists in other part of code. > -----Original Message----- > From: Laszlo Ersek [mailto:ler...@redhat.com] > Sent: Friday, September 22, 2017 11:29 PM > To: Wang, Jian J <jian.j.w...@intel.com>; edk2-devel@lists.01.org > Cc: Zeng, Star <star.z...@intel.com>; Dong, Eric <eric.d...@intel.com>; Yao, > Jiewen <jiewen....@intel.com>; Kinney, Michael D > <michael.d.kin...@intel.com>; Justen, Jordan L <jordan.l.jus...@intel.com>; > Wolman, Ayellet <ayellet.wol...@intel.com> > Subject: Re: [PATCH v2 6/6] OvmfPkg/QemuVideoDxe: Bypass NULL pointer > detection during VBE SHIM installing > > On 09/22/17 13:50, Laszlo Ersek wrote: > > This patch looks great to me, I would like to request a few small > > updates: > > > > On 09/21/17 07:20, Jian J Wang wrote: > >> QemuVideoDxe driver will install VBE SHIM into page 0. If NULL pointer > > > > (1) please replace the word "install" with "link". > > > > The VBE Shim is technically installed into the "real-mode" C segment, > > only the int 0x10 vector lives in page 0. > > > >> detection is enabled, this driver will fail to load. NULL pointer detection > >> bypassing code is added to prevent such problem during boot. > >> > >> Please note that Windows 7 will try to access VBE SHIM during boot if it's > >> installed, and then cause boot failure. This can be fixed by setting BIT7 > >> of PcdNullPointerDetectionPropertyMask to disable NULL pointer detection > >> after EndOfDxe. As far as we know, there's no other OSs has such issue. > > > > This is not a request, just a comment: I verified the default value in > > the .dec, and I see it is 0. So there's no need to post an additional > > patch for the OVMF DSC files, in order to set BIT7. > > Actually, let me take a step back, and re-think the necessity of all > this work for QemuVideoDxe! > > The facts are: > > (1) The *only* purpose of the VBE Shim is to allow Windows 7 to boot in > pure UEFI mode (i.e. without a CSM). > > (2) If I understand correctly, you guys have verified that Windows 7 > cannot boot with the page0 protection enabled, *regardless* of what we > do in QemuVideoDxe. Can you confirm this please? > > With the above in mind, let's consider the effects of the > "PcdNullPointerDetectionPropertyMask" bits: > > * BIT0 clear: > - The page0 protection is completely disabled. > - This patch does nothing, in effect. > - The VBE Shim works. > - Windows 7 boots. > > * BIT0 set, BIT7 also set: > - The page0 protection is disabled in the DXE core at the end of DXE. > - This patch does nothing, in effect. > - The VBE Shim works, because it is a UEFI driver, and it connects its > devices (and installs the shim) after End-of-Dxe, at which point > page0 protection is no longer in effect. > - Windows 7 boots fine, again because it is loaded after End-of-Dxe. > > * BIT0 set, BIT7 clear: > - The page0 protection is never disabled until the OS (loader) > installs its own page tables. > - This patch enables the VBE Shim to work, by temporarily disabling > page0 protection. > - However, Windows 7 will fail to boot nonetheless, because it cannot > cope with page0 protection. (This is fact (2).) > > Now, if you consider fact (1) as well: given that Windows 7 cannot boot > with page0 protection enabled *anyway*, why mess with the VBE Shim at > all? > > How about the following patch instead: > > > diff --git a/OvmfPkg/QemuVideoDxe/VbeShim.c > b/OvmfPkg/QemuVideoDxe/VbeShim.c > > index e45a08e8873f..8ba5522cde3c 100644 > > --- a/OvmfPkg/QemuVideoDxe/VbeShim.c > > +++ b/OvmfPkg/QemuVideoDxe/VbeShim.c > > @@ -75,6 +75,20 @@ InstallVbeShim ( > > UINTN Printed; > > VBE_MODE_INFO *VbeModeInfo; > > > > + if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & (BIT0|BIT7)) == > > BIT0) > { > > + DEBUG (( > > + DEBUG_WARN, > > + "%a: page 0 protected, not installing VBE shim\n", > > + __FUNCTION__ > > + )); > > + DEBUG (( > > + DEBUG_WARN, > > + "%a: page 0 protection prevents Windows 7 from booting anyway\n", > > + __FUNCTION__ > > + )); > > + return; > > + } > > + > > Segment0 = 0x00000; > > SegmentC = 0xC0000; > > SegmentF = 0xF0000; > > Thanks! > Laszlo _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
