Re: [PATCH 0/4] Fix runtime issue in XenBusDxe when compiled with GCC5

[Laszlo Ersek <lersek@redhat.com>]

On 02/21/17 18:53, Anthony PERARD wrote:
> On Tue, Feb 21, 2017 at 06:07:15PM +0100, Laszlo Ersek wrote:
>> CC Rebecca & Konrad
>>
>> On 02/21/17 17:39, Anthony PERARD wrote:

[snip]

>>> So, should I add EFIAPI to XenStoreVSPrint, as it is using VA_COPY?
>>>
>>
>> Hm, please help me jog my memory...
>>
>> If I remember correctly, this is still a GCC bug, one that we suppressed for gcc-6.2 with your patch as follows:
> 
> Yes.
> 
>>> commit 432f1d83f77acf92d52ef18d2cee6dbf7c5b9b86
>>> Author: Anthony PERARD <anthony.perard@citrix.com>
>>> Date:   Tue Dec 6 12:03:25 2016 +0000
>>>
>>>     OvmfPkg/build.sh: Use GCC49 toolchains with GCC 6.[0-2]
>>>
>>>     The goal of the patch is to avoid using -flto with GCC 6.0 to 6.2.
>>>
>>>     This is to workaround a GCC bug:
>>>     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70955
>>>
>>>     Contributed-under: TianoCore Contribution Agreement 1.0
>>>     Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
>>>     Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>>>     Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
>>>
>>> diff --git a/OvmfPkg/build.sh b/OvmfPkg/build.sh
>>> index 95fe8fb07647..b6e936056ca0 100755
>>> --- a/OvmfPkg/build.sh
>>> +++ b/OvmfPkg/build.sh
>>> @@ -102,7 +102,7 @@ case `uname` in
>>>        4.8.*)
>>>          TARGET_TOOLS=GCC48
>>>          ;;
>>> -      4.9.*)
>>> +      4.9.*|6.[0-2].*)
>>>          TARGET_TOOLS=GCC49
>>>          ;;
>>>        *)
>>
>> Do I understand correctly that the gcc bug has not been fixed in
>> gcc-6.3, and -- because we don't suppress it for gcc-6.3 as the
>> above expression does not match -- it causes problems again?
> 
> The bug describe in the GCC bugzilla is probably fix, but the
> test-case does not make use of __builtin_ms_va_copy.

:/

> 
>> You also mention gcc-5.4 as problematic. I think we haven't
>> received such reports about gcc-5 versions up to and including
>> gcc-5.3 (that's why GCC5 is the default selection in
>> "OvmfPkg/build.sh"). Do you mean that the gcc bug has now been
>> "backported" from the gcc-6 series to the gcc-5 series (starting
>> with gcc-5.4)?

> 
> I don't know the state of gcc-5.0 to gcc-5.3, I have never tested -flto
> with gcc-5.x (until now), I would say they are also problematic until
> proven otherwise.

When we enabled GCC5, it definitely worked for at least one gcc release,
with -flto. (-flto is the default for DEBUG and RELEASE builds with
GCC5; NOOPT disables -Os and -flto.)

> 
>> If that's the case, then I suggest flipping "OvmfPkg/build.sh" from
>> black-listing gcc versions for -flto to white-listing. In other
>> words, assume that -flto is generally broken with GCC, except for a
>> few known versions: 5.0 through 5.3 inclusive. Those versions
>> should trigger the use of the GCC5 toolchain, and everything else
>> (5.4+, 6.*, 4.9.*) should use GCC49.
>>
>> I don't feel comfortable about adding EFIAPI to XenStoreVSPrint
>> just because it takes a VA_LIST parameter -- note: it is *not* a
>> varargs function itself! --; the same issue might hit elsewhere in
>> the edk2 tree at any time, outside of OvmfPkg too.
> 
> From the different tests I've done, I feel more like VA_COPY might be
> the issue, but I don't know how __builtin_ms_va_* are supposed to be
> used.

If I recall correctly, from the upstream GCC bug, the problem is that
__builtin_va_list does not track internally whether it was created in an
msabi or sysvabi function, and therefore the va_* functions cannot be
used transparently on it. Instead, when va_list is accessed, the
accessor builtins seem to apply the currently executing function's
calling convetion to va_list. (Even if the creation context of va_list
was different.)

> 
>> Would the gcc white-listing work for you?
>>
>> Note that the white-listing would practically undo Konrad's commit
>> 2667ad40919a ("OvmfPkg/build.sh: Make GCC5 the default toolchain,
>> catch GCC43 and earlier", 2016-11-23), but given the recent gcc
>> developments (gcc-6.3 has not fixed the gcc bug, and the bug has
>> even surfaced in gcc-5.4), I think it would be justified.
> 
> Do be honnest, I don't think the toolchain GCC5 has ever been tested
> with gcc-5.x and the module XenBusDxe. I think most people that want to
> start OVMF under Xen are likely to build it with gcc-4.9 or already had
> gcc-6.x when OVMF switch to the GCC5 toolchain by default.
> 

Okay... I'm equally fine if we just say "given that GCC is broken like
this, we hereby require all functions that take a variable argument
list, *or* a VA_LIST parameter, to be EFIAPI". (The first part of the
requirement already exists.)

But in this case, the full edk2 codebase has to be grepped for
VA_LIST-taking functions, and all of them must be flipped to EFIAPI, if
they currently aren't EFIAPI. Covering just XenStoreVSPrint() seems
incomplete. (Note: CryptoPkg/Library/OpensslLib is an exception.)

Also, in this case, your commit 432f1d83f77a should likely be reverted.
(Because we are ultimately giving in to the gcc bug.)

Thanks
Laszlo