Re: [edk2-devel] [PATCH v9 08/46] UefiCpuPkg: Implement library support for VMGEXIT

["Laszlo Ersek" <lersek@redhat.com>]

On 07/06/20 22:03, Tom Lendacky wrote:
> On 7/2/20 2:04 AM, Dong, Eric wrote:
>> Hi Tom,
> 
> Hi Eric,
> 
>>
>> We have root cause this Mac file format issue. The patch mail from your side include extra two "=0D=0D" , and our test tool convert them to "\r\r". This is Mac file line ending format. So this issue been reported. We have updated our tool to handle this special case.
> 
> Good to know, thanks!
> 
>>
>> With that change, now I met below error when use VS2015 tool chain. Can you help to fix it?
>>
>> Building ... g:\edk2-open-source\edk2\MdePkg\Library\PeiCoreEntryPoint\PeiCoreEntryPoint.inf [X64]
>> PeCoffLoaderEx.c
>> g:\edk2-open-source\edk2\OvmfPkg\Library\VmgExitLib\VmgExitVcHandler.c(386): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
>> NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 14.0\Vc\bin\x86_amd64\cl.exe"' : return code '0x2'

This is for the line

      Displacement *= (1 << Ext->Sib.Scale);

from

  [edk2-devel] [PATCH v9 17/46]
  OvmfPkg/VmgExitLib: Add support for NPF NAE events (MMIO)

> 
> Yup, looks like that needs to be a "1ULL <<" instead of "1 <<".
> I have verified that fixes the issue.

I disagree.

At that point, Displacement is of type INT64, and it may well be a
negative value. We definitely want to multiply it by a signed int
(values 1, 2, 4, 8).

I commented on this before. Please see:

(i) my comment block (10) here -- especially comment (10c):

  https://edk2.groups.io/g/devel/message/60144

(alternative link:
<http://mid.mail-archive.com/169e44cb-2c1c-6d9a-342a-2a1f618e3753@redhat.com>)

(ii) and my comment here:

  https://edk2.groups.io/g/devel/message/60146

(alternative link:
<http://mid.mail-archive.com/139ce789-b938-c8b9-030e-c1b6c67e47ea@redhat.com>).


The compiler warning is well-meaning, but unnecessary. A 64-bit shift is
*NOT* intended. We want to end up with one of the signed int (aka INT32)
values 1, 2, 4 or 8. And then multiply the INT64 Displacement with that
value. For the multiplication, the INT32 value 1, 2, 4 or 8 will be
implicitly converted to INT64. That's entirely intentional.

If we want to suppress the warning, while keeping the logic intact, we
should employ an explicit cast:

  Displacement *= (INT64)(1 << Ext->Sib.Scale);

> 
> One thing I noticed is that the 32-bit builds
> (PlatformCI_OvmfPkg_Windows_VS2019_PR, Platform_CI OVMF_IA32_NOOPT and
> Platform_CI OVMF_IA32X64_NOOPT) encounter an error:
> 
> ERROR - Linker #2001 from SecMain.lib(SecMain.obj) :   unresolved external symbol __allshl
> ERROR - Linker #1120 from d:\a\1\s\Build\Ovmf3264\NOOPT_VS2019\IA32\OvmfPkg\Sec\SecMain\DEBUG\SecMain.dll : fatal   1 unresolved externals
> ERROR - Compiler #1077 from NMAKE : fatal   '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\bin\Hostx86\x86\link.exe"' : return code '0x460'
> 
> Any idea what is causing this error?

A left-shift operator (<<) applied to a 64-bit operand is somehow
finding its way into the 32-bit SEC build.

That is indeed wrong (for such cases, we're supposed to use LShiftU64()
from BaseLib).

What I don't understand however is that all of the "<<" operator uses,
on 64-bit operands, should already be limited to code that is *only*
built for X64!

For example, with this series applied, SecMain in OVMF consumes
"UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf".
And the latter consumes VmgExitLib.

But VmgExitLib is resolved to
"UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.inf", in the IA32 and
IA32X64 DSC files. This Null instance contains no left-shifts.

Therefore any << operators, applied to 64-bit operands, present in
"OvmfPkg/Library/VmgExitLib", should never be compiled for IA32 and IA32X64.

So I don't know where the problematic "<<" comes from. It does not come
from VmgExitLib, as far as I can tell.

Thanks,
Laszlo