Re: [PATCH] MdePkg/BaseIoLibIntrinsicArmVirt ARM: avoid double word loads and stores

[Laszlo Ersek <lersek@redhat.com>]

On 11/07/18 17:47, Philippe Mathieu-Daudé wrote:
> On 7/11/18 14:38, Ard Biesheuvel wrote:
>> On 7 November 2018 at 14:13, Ard Biesheuvel
>> <ard.biesheuvel@linaro.org> wrote:
>>> BaseIoLibIntrinsicArmVirt was created to prevent LTO from merging
>>> accesses to MMIO regions, resulting in instructions with multiple
>>> output registers that KVM on ARM cannot emulate (since the exception
>>> syndrome information that KVM relies on can only describe a single
>>> output register)
>>>
>>> However, using double word loads on ARM amounts to the same thing,
>>> and so code that relies on doing 64-bit MMIO to regions that are
>>> emulated under KVM (such as the GICv3 TYPER register) will still
>>> suffer from the original issue.
>>>
>>> So replace ldrd and strd with equivalent two instruction sequences.
>>>
>>> Contributed-under: TianoCore Contribution Agreement 1.1
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>
>> Please consider this patch with the hunk below appended
>>
>>> ---
>>>   MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.S | 6 ++++--
>>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.S
>>> b/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.S
>>> index 3ad22bd5706d..0d802d6928d6 100644
>>> --- a/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.S
>>> +++ b/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.S
>>> @@ -125,7 +125,8 @@ ASM_PFX(MmioWrite32Internal):
>>>   //  @return The value read.
>>>   //
>>>   ASM_PFX(MmioRead64Internal):
>>> -  ldrd    r0, r1, [r0]
>>> +  ldr     r1, [r0, #4]
>>> +  ldr     r0, [r0]
> 
> Ard remembered me UEFI is little-endian only :)
> Thus this code is safe.
> 
> With the hunk appended:
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

So, this is for 32-bit ARM only, IIUC. (Supported also by the pathnames
of the files being modified.)

The commit message makes sense, and the patch looks plausible. (I didn't
go to the ARM ARM to verify the syntax, and I admit I don't know the
syntax without looking it up.)

Acked-by: Laszlo Ersek <lersek@redhat.com>

Thanks!
Laszlo


> 
>>>     dmb
>>>     bx      lr
>>>
>>> @@ -141,5 +142,6 @@ ASM_PFX(MmioRead64Internal):
>>>   //
>>>   ASM_PFX(MmioWrite64Internal):
>>>     dmb     st
>>> -  strd    r2, r3, [r0]
>>> +  str     r2, [r0]
>>> +  str     r3, [r0, #4]
>>>     bx      lr
>>
>> diff --git a/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.asm
>> b/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.asm
>> index e1a3d68a430c..deba8c1f0c59 100644
>> --- a/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.asm
>> +++ b/MdePkg/Library/BaseIoLibIntrinsic/Arm/ArmVirtMmio.asm
>> @@ -127,7 +127,8 @@ MmioWrite32Internal
>>   ;  @return The value read.
>>   ;
>>   MmioRead64Internal
>> -  ldrd    r0, r1, [r0]
>> +  ldr     r1, [r0, #4]
>> +  ldr     r0, [r0]
>>     dmb
>>     bx      lr
>>
>> @@ -143,7 +144,8 @@ MmioRead64Internal
>>   ;
>>   MmioWrite64Internal
>>     dmb     st
>> -  strd    r2, r3, [r0]
>> +  str     r2, [r0]
>> +  str     r3, [r0, #4]
>>     bx      lr
>>
>>     END
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