Re: [PATCH] ArmVirtPkg/ArmVirtQemu: use non-accelerated CopyMem for VariableRuntimeDxe

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 15 November 2017 at 13:51, Laszlo Ersek <lersek@redhat.com> wrote:
> On 11/14/17 11:22, Ard Biesheuvel wrote:
>> The VariableRuntimeDxe driver may use CopyMem () on NOR flash regions,
>> assuming such regions always have full memory semantics. Given that
>> those regions cannot be mapped as ordinary memory on ARM (due to the
>> fact that the NOR flash requires device semantics while in write mode)
>> this prevents us from using BaseMemoryLibOptDxe in VariableRuntimeDxe,
>> since it may use unaligned accesses and/or DC ZVA instructions, both
>> of which are incompatible with mappings using device semantics.
>>
>> Note that there is no way we can work around this by changing the
>> mapping type between 'memory' and 'device' when switching from read to
>> write mode and back, because the runtime mapping is created by the OS,
>> and cannot be changed at will.
>>
>> So let's just switch to the unaccelerated version of BaseMemoryLib which
>> does not have the same problem.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  ArmVirtPkg/ArmVirtQemu.dsc       | 2 ++
>>  ArmVirtPkg/ArmVirtQemuKernel.dsc | 2 ++
>>  2 files changed, 4 insertions(+)
>>
>> diff --git a/ArmVirtPkg/ArmVirtQemu.dsc b/ArmVirtPkg/ArmVirtQemu.dsc
>> index 8a60b61f2aa6..7b220d6e3c31 100644
>> --- a/ArmVirtPkg/ArmVirtQemu.dsc
>> +++ b/ArmVirtPkg/ArmVirtQemu.dsc
>> @@ -261,6 +261,8 @@ [Components.common]
>>    MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf {
>>      <LibraryClasses>
>>        NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
>> +      # don't use unaligned CopyMem () on the UEFI varstore NOR flash region
>> +      BaseMemoryLib|MdePkg/Library/BaseMemoryLib/BaseMemoryLib.inf
>>    }
>>  !if $(SECURE_BOOT_ENABLE) == TRUE
>>    MdeModulePkg/Universal/SecurityStubDxe/SecurityStubDxe.inf {
>> diff --git a/ArmVirtPkg/ArmVirtQemuKernel.dsc b/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> index 9a31ec93ca06..7c032e1b07e0 100644
>> --- a/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> +++ b/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> @@ -252,6 +252,8 @@ [Components.common]
>>    MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf {
>>      <LibraryClasses>
>>        NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
>> +      # don't use unaligned CopyMem () on the UEFI varstore NOR flash region
>> +      BaseMemoryLib|MdePkg/Library/BaseMemoryLib/BaseMemoryLib.inf
>>    }
>>  !if $(SECURE_BOOT_ENABLE) == TRUE
>>    MdeModulePkg/Universal/SecurityStubDxe/SecurityStubDxe.inf {
>>
>
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>
> Given that we've never seen the symptom reported by Shannon, I must
> think Shannon is using some kind of new hardware. May I ask what
> hardware that is?
>

I think this is equally likely to occur in any KVM hardware
virtualization context. It is simply a regression after moving to the
OptDxe BaseMemoryLib flavor.