Re: [Xen-devel] [edk2] [PATCH v3 21/27] Ovmf/Xen: add ARM and AArch64 support to XenBusDxe

[Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>]

On 4 February 2015 at 21:10, Jordan Justen <jordan.l.justen@xxxxxxxxx> wrote:
> On 2015-02-03 11:20:06, Ard Biesheuvel wrote:
>> This patch adds support to XenBusDxe for executing on ARM and AArch64
>> machines (the former only when built with GCC).
>>
>> Contributed-under: TianoCore Contribution Agreement 1.0
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> ---
>>  OvmfPkg/XenBusDxe/AtomicsGcc.c  | 44 
>> +++++++++++++++++++++++++++++++++++++++++
>>  OvmfPkg/XenBusDxe/XenBusDxe.inf |  3 +++
>>  2 files changed, 47 insertions(+)
>>  create mode 100644 OvmfPkg/XenBusDxe/AtomicsGcc.c
>>
>> diff --git a/OvmfPkg/XenBusDxe/AtomicsGcc.c b/OvmfPkg/XenBusDxe/AtomicsGcc.c
>> new file mode 100644
>> index 000000000000..a0bdcbf67440
>> --- /dev/null
>> +++ b/OvmfPkg/XenBusDxe/AtomicsGcc.c
>> @@ -0,0 +1,44 @@
>> +/** @file
>> +  Arch-independent implementations of XenBusDxe atomics using GCC __builtins
>> +
>> +  Copyright (C) 2014, Linaro Ltd.
>> +
>> +  This program and the accompanying materials
>> +  are licensed and made available under the terms and conditions of the BSD 
>> License
>> +  which accompanies this distribution.  The full text of the license may be 
>> found at
>> +  http://opensource.org/licenses/bsd-license.php
>> +
>> +  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
>> +  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR 
>> IMPLIED.
>> +
>> +**/
>> +
>> +UINT16
>> +EFIAPI
>> +InternalSyncCompareExchange16 (
>> +  IN      volatile UINT16           *Value,
>> +  IN      UINT16                    CompareValue,
>> +  IN      UINT16                    ExchangeValue
>
> Can you instead get InterlockedCompareExchange16 added to
> BaseSynchronizationLib?
>
> http://permalink.gmane.org/gmane.comp.bios.tianocore.devel/10470
>

That would seem the obvious choice, but sadly, BaseSynchronizationLib
is not implemented at all for ARM and AArch64 (Some .c files are there
but they are just unsynchronized C implementations)

So in order to do this properly, I would need to
- implement the existing functions for ARM and AArch64
- implement InterlockedCompareExchange16 () for all existing
architectures, including IPF, which I don't have a clue about, and
toolchains I don't have access to
- implement TestAndClearBit () for all architectures, or create a
BaseBitopsLib and implement it there

I now understand how Anthony ended up doing it like this in the first place :-)

Anyway, we are all invested in improving the code base, so I am happy
to do my part, but I would need help from others to implement the
pieces that are really not feasible for me to take on.

-- 
Ard.

>> +  )
>> +{
>> +  return __sync_val_compare_and_swap_2 (Value, CompareValue, ExchangeValue);
>> +}
>> +
>> +INT32
>> +EFIAPI
>> +TestAndClearBit (
>> +  IN INT32            Bit,
>> +  IN volatile VOID    *Address
>> +  )
>> +{
>> +  //
>> +  // Calculate the effective address relative to 'Address' based on the
>> +  // higher order bits of 'Bit'. Use signed shift instead of division to
>> +  // ensure we round towards -Inf, and end up with a positive shift in 
>> 'Bit',
>> +  // even if 'Bit' itself is negative.
>> +  //
>> +  Address += (Bit >> 5) * sizeof(INT32);
>> +  Bit &= 31;
>> +
>> +  return (__sync_fetch_and_and_4 (Address, ~(1U << Bit)) & (1U << Bit)) != 
>> 0;
>> +}
>> diff --git a/OvmfPkg/XenBusDxe/XenBusDxe.inf 
>> b/OvmfPkg/XenBusDxe/XenBusDxe.inf
>> index 31553ac5a64a..949ec0a0c732 100644
>> --- a/OvmfPkg/XenBusDxe/XenBusDxe.inf
>> +++ b/OvmfPkg/XenBusDxe/XenBusDxe.inf
>> @@ -54,6 +54,9 @@
>>    X64/InterlockedCompareExchange16.nasm
>>    X64/TestAndClearBit.nasm
>>
>> +[Sources.AARCH64, Sources.ARM]
>> +  AtomicsGcc.c | GCC
>> +
>>  [LibraryClasses]
>>    UefiDriverEntryPoint
>>    UefiBootServicesTableLib
>> --
>> 1.8.3.2
>>
>>
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