Re: ASSERT in QemuVideoDxe driver during reset

["Yao, Jiewen" <jiewen.yao@intel.com>]

HI Laszlo
Would you please clarify Window 7 is "UEFI Window 7" or "Legacy Window 7" ?

Thank you
Yao Jiewen

From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Laszlo Ersek
Sent: Wednesday, September 6, 2017 9:19 PM
To: Yao, Jiewen <jiewen.yao@intel.com>; Wang, Jian J <jian.j.wang@intel.com>; Justen, Jordan L <jordan.l.justen@intel.com>
Cc: Kinney, Michael D <michael.d.kinney@intel.com>; edk2-devel@lists.01.org
Subject: Re: [edk2] ASSERT in QemuVideoDxe driver during reset

On 09/06/17 13:16, Yao, Jiewen wrote:
> HI
> I think the NULL detection feature should *never* break any existing platform.
> No real function should be skipped for PcdNullDetection.
>
> If InstallVbeShim () is needed for CSM, we should always call InstallVbeShim() for CSM.

Wait a second please -- the VBE shim exists so that we can boot Windows7
on OVMF *without* the CSM.

If you build OVMF with the SeaBIOS CSM, then Windows7 will simply boot
in legacy mode, and work as usual.

But if you build OVMF without a CSM, then Windows7 will break mid-way
into booting. The reason is that Windows7 insists on some legacy VBE
services even if it is booted on a pure UEFI system. By faking a minimal
set of legacy VBE services, Windows7 boots just fine in UEFI mode.

(Windows 7 doesn't actually execute the VBE services' real mode code on
the processor; instead the code is interpreted in a software emulator.)

The dependency on legacy VBE services is arguably a bug of UEFI Windows
7. One way to work it around is to build OVMF with the SeaBIOS CSM, and
just boot Windows7 in legacy mode.

However, in some environments, building OVMF with a CSM is not
desirable. That's why we created the VBE shim. It is a terrible hack,
but it fits the Windows7 bug just fine.

> I suggest below options:
>
> 1)       In OVMF, if CSM is enabled, disable PcdNullDetection.
>
> 2)       In OVMF, if any driver need access first 4K page, the code should explicit call SetAttribute(0, 4K, READ|WRITE), if PcdNullDetection is enabled.
>
> Either #1 or #2 is OK.
> #1 is simple, and #2 can help detect more potential issue.

* Regarding #1:

As I wrote above, the VBE shim is mutually exclusive with CSM:

  //
  // The allocation request may fail, eg. if LegacyBiosDxe has already run.
  //

So, if the CSM was built into OVMF, then the VBE shim is already skipped
at boot.


* Regarding #2:

I agree that *temporarily* enabling R/W access to page zero, using the
appropriate DXE service, could make the VBE Shim installation work.

However, making page 0 unaccessible right after, could very easily break
Windows 7, if some component of Windows 7 accessed page 0, before
setting up new page tables. After all, in the InstallVbeShim() function,
we write stuff to page 0 *because* Windows 7 will read it from there.


* If it is more convenient than a "--pcd=..." option for "build", we can
also add another build-time macro to the OVMF DSC files, called
NULL_DETECT_ENABLE. This would then turn on the PcdNullDetection bits,
for "-D NULL_DETECT_ENABLE".

NULL_DETECT_ENABLE should default to FALSE.


* Independently: if the CSM too conflicts with the null detection
feature, then we should probably report an "unsupported configuration"
error for the following case:

  -D NULL_DETECT_ENABLE -D CSM_ENABLE

But, I don't know if triggering build errors, with custom error
messages, is possible in DSC files.

Thanks
Laszlo


> From: Laszlo Ersek [mailto:lersek@redhat.com]
> Sent: Wednesday, September 6, 2017 6:21 PM
> To: Wang, Jian J <jian.j.wang@intel.com<mailto:jian.j.wang@intel.com>>; Justen, Jordan L <jordan.l.justen@intel.com<mailto:jordan.l.justen@intel.com>>; Yao, Jiewen <jiewen.yao@intel.com<mailto:jiewen.yao@intel.com>>
> Cc: edk2-devel@lists.01.org<mailto:edk2-devel@lists.01.org>; Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
> Subject: Re: ASSERT in QemuVideoDxe driver during reset
>
> On 09/06/17 10:15, Wang, Jian J wrote:
>> Hi guys,
>>
>> I found an ASSERT issue in function InstallVbeShim() in QemuVideoDxe
>> driver during reset. The assert statement is like below.
>>
>>     ASSERT (Int0x10->Segment == 0x0000);
>>     ASSERT (Int0x10->Offset  == 0x0000);
>>
>> This happened after I have enabled NULL pointer access detection
>> feature, in which page 0 (4K)  is disabled.
>
> The NULL pointer access detection conflicts with the VBE shim. For
> installing the VBE shim, OVMF has to write to page 0, on purpose.
>
> Please see commit 90803342b1b6 ("OvmfPkg: QemuVideoDxe: Int10h stub for
> Windows 7 & 2008 (stdvga, QXL)", 2014-05-20).
>
>> And because of page 0 disabled, I have to skip the memory clearing for
>> page 0 in DXE core.
>
> By doing that, you invalidate the following comment in said OVMF commit:
>
> +    //
> +    // We managed to allocate the page at zero. SVN r14218 guarantees that it
> +    // is NUL-filled.
> +    //
> +    ASSERT (Int0x10->Segment == 0x0000);
> +    ASSERT (Int0x10->Offset  == 0x0000);
>
> Because SVN r14218 was what added the clearing of page 0 to the DXE
> core. (Expressed as a git commit: d436d5ca0936.)
>
>> Otherwise it will cause page fault exception there. It seems that QEMU
>> may clear all its memory at startup. Skipping the action of clearing
>> page 0 in core won't cause ASSERT issue in QemuVideoDxe, for the first
>> time boot. But QemuVideoDxe will write int10 vector at memory 0x10 and
>> QEMU will not clear all its memory during warm boot. ASSERT will be
>> triggered after reset.
>
> QEMU does not clear guest RAM at reboot (or S3 resume).
>
>> It's easy to fix this issue but there're some subtle situations which
>> I'm not quite certain. I'd like your opinions for them.
>>
>> Here're my thoughts on several solutions:
>> a) Remove the ASSERT statement in InstallVbeShim().
>
> Not a good idea.
>
>> But I'm sure if it is safe to do so because I don't quite understand
>> the purpose of the ASSERT.
>
> The ASSERT expresses our belief that, after we manage to *allocate* page
> 0, we can freely write the real mode Int0x10 vector there, without
> overwriting anything else.
>
>> b) Instead of skipping clearing page 0, enable it, do clearing and
>> then disable it. The problem here is that CPU arch protocol is not
>> ready at that time. I have to "manually" do page operation, which
>> might be non-portable and a little bit odd in DXE core.
>> c) Move code clearing page 0 from DXE core to another place wherever
>> appropriate, like DxeIpl or cpu driver. But I think there's a good
>> reason to put code there before.
>
> I'm not sure how the NULL pointer access detection is enabled. Is it
> enabled by a PCD?
>
> Hm, yes, it seems, from your patch
>
>   [edk2] [PATCH 1/2] Implement NULL pointer detection for EDK-II Core
>
> that the PCD is called
>
>   gEfiMdeModulePkgTokenSpaceGuid.PcdNullPointerDetection
>
> I can see in that patch that the DXE core is updated *not* to clear page
> 0 if the PCD is set to TRUE. That makes sense.
>
> However, OVMF should also be updated to skip the VBE shim installation
> if the PCD is set to TRUE. Namely, if the PCD is set to TRUE, then we
> simply cannot put the legacy Int0x10 vector in place, so we shouldn't
> even try.
>
> Please extend your original patch set with a patch for OvmfPkg. In the
> file "OvmfPkg/QemuVideoDxe/Driver.c", locate the InstallVbeShim() call
> site:
>
>> #if defined MDE_CPU_IA32 || defined MDE_CPU_X64
>>   if (Private->Variant == QEMU_VIDEO_BOCHS_MMIO ||
>>       Private->Variant == QEMU_VIDEO_BOCHS) {
>>     InstallVbeShim (Card->Name, Private->GraphicsOutput.Mode->FrameBufferBase);
>>   }
>> #endif
>
> and please skip the call if the PCD is set to TRUE:
>
>> #if defined MDE_CPU_IA32 || defined MDE_CPU_X64
>>   if (!FeaturePcdGet (PcdNullPointerDetection) &&
>>       (Private->Variant == QEMU_VIDEO_BOCHS_MMIO ||
>>        Private->Variant == QEMU_VIDEO_BOCHS)) {
>>     InstallVbeShim (Card->Name, Private->GraphicsOutput.Mode->FrameBufferBase);
>>   }
>> #endif
>
> Now, the consequence of this would be a working OVMF build, but it would
> also break booting UEFI Windows 7 on OVMF. Therefore, please append
> *another* patch to your patch set, setting the Feature PCD to FALSE in
> all three OVMF DSC files:
>
> - OvmfPkg/OvmfPkgIa32.dsc
> - OvmfPkg/OvmfPkgIa32X64.dsc
> - OvmfPkg/OvmfPkgX64.dsc
>
> We cannot break Windows 7 booting in upstream OVMF. If someone really
> wants the null pointer detection in OVMF (and doesn't need Windows 7 for
> sure), they can patch the DSC files, or else pass the
>
>   --pcd=gEfiMdeModulePkgTokenSpaceGuid.PcdNullPointerDetection=TRUE
>
> option to the "build" utility.
>
> ... I'm now seeing in the original discussion that perhaps you will
> introduce a bitmap instead (one bit per firmware phase). That's OK with
> me; please reinterpret all of the above with the "DXE bit" then.
>
> When you post the next version of the patch set, please CC Jordan and
> myself on the OvmfPkg patches.
>
> Thank you!
> Laszlo
>

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