Re: [PATCH] ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel

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

On 22 August 2017 at 18:02, Laszlo Ersek <lersek@redhat.com> wrote:
> On 08/22/17 18:30, Ard Biesheuvel wrote:
>> One of the reasons for introducing virtio-gpu support to OvmfPkg and
>> ArmVirtpkg was the fact that under KVM virtualization on ARM, the
>> legacy VGA cannot be used reliably. This is due to an implementation
>> detail of QEMU+KVM, which remaps cached host memory into the guest
>> address space as a framebuffer behind a PCI BAR. Given that the purpose
>> of a memory mapped framebuffer is its side effects, such BARs should
>> never be mapped cacheable in the guest, and the mismatched attributes
>> between host and guest result in a loss of coherency, visible as
>> corruption in the framebuffer image.
>>
>> This issue does not occur under TCG emulation, nor did we expect it to
>> actually bring down the guest under KVM, and so it was deemed harmless
>> to keep support for the VGA device as well. However, as it turns out,
>> the fact that the framebuffer BAR is mapped using device semantics by
>> default may result in unalignment faults when we use the ordinary string
>> copy routines on the contents. In theory, we could work around this by
>> remapping the BAR as write combining, but it appears the generic PCI
>> bus driver does not actually implement this.
>>
>> So let's remove the QemuVideoDxe driver altogether. This may result
>> in loss of functionality for use cases that rely on the framebuffer
>> to be directly addressable (such as EFIFB), but given that this never
>> worked reliably under KVM in the first place, let's not let that stop
>> us from dropping support for it.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  ArmVirtPkg/ArmVirtQemu.dsc           | 2 --
>>  ArmVirtPkg/ArmVirtQemuFvMain.fdf.inc | 1 -
>>  ArmVirtPkg/ArmVirtQemuKernel.dsc     | 2 --
>>  3 files changed, 5 deletions(-)
>>
>> diff --git a/ArmVirtPkg/ArmVirtQemu.dsc b/ArmVirtPkg/ArmVirtQemu.dsc
>> index e23a6d17bc44..2e6e76224987 100644
>> --- a/ArmVirtPkg/ArmVirtQemu.dsc
>> +++ b/ArmVirtPkg/ArmVirtQemu.dsc
>> @@ -57,7 +57,6 @@
>>    BootLogoLib|MdeModulePkg/Library/BootLogoLib/BootLogoLib.inf
>>    PlatformBootManagerLib|ArmVirtPkg/Library/PlatformBootManagerLib/PlatformBootManagerLib.inf
>>    CustomizedDisplayLib|MdeModulePkg/Library/CustomizedDisplayLib/CustomizedDisplayLib.inf
>> -  FrameBufferBltLib|MdeModulePkg/Library/FrameBufferBltLib/FrameBufferBltLib.inf
>>    QemuBootOrderLib|OvmfPkg/Library/QemuBootOrderLib/QemuBootOrderLib.inf
>>    FileExplorerLib|MdeModulePkg/Library/FileExplorerLib/FileExplorerLib.inf
>>    PciPcdProducerLib|ArmVirtPkg/Library/FdtPciPcdProducerLib/FdtPciPcdProducerLib.inf
>> @@ -357,7 +356,6 @@
>>    #
>>    # Video support
>>    #
>> -  OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
>>    OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
>>    OvmfPkg/PlatformDxe/Platform.inf
>>
>> diff --git a/ArmVirtPkg/ArmVirtQemuFvMain.fdf.inc b/ArmVirtPkg/ArmVirtQemuFvMain.fdf.inc
>> index 237b2d03a714..3194aa3edc8e 100644
>> --- a/ArmVirtPkg/ArmVirtQemuFvMain.fdf.inc
>> +++ b/ArmVirtPkg/ArmVirtQemuFvMain.fdf.inc
>> @@ -167,7 +167,6 @@ READ_LOCK_STATUS   = TRUE
>>    #
>>    # Video support
>>    #
>> -  INF OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
>>    INF OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
>>    INF OvmfPkg/PlatformDxe/Platform.inf
>>
>> diff --git a/ArmVirtPkg/ArmVirtQemuKernel.dsc b/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> index aa01debfda69..69de887277cb 100644
>> --- a/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> +++ b/ArmVirtPkg/ArmVirtQemuKernel.dsc
>> @@ -57,7 +57,6 @@
>>    BootLogoLib|MdeModulePkg/Library/BootLogoLib/BootLogoLib.inf
>>    PlatformBootManagerLib|ArmVirtPkg/Library/PlatformBootManagerLib/PlatformBootManagerLib.inf
>>    CustomizedDisplayLib|MdeModulePkg/Library/CustomizedDisplayLib/CustomizedDisplayLib.inf
>> -  FrameBufferBltLib|MdeModulePkg/Library/FrameBufferBltLib/FrameBufferBltLib.inf
>>    QemuBootOrderLib|OvmfPkg/Library/QemuBootOrderLib/QemuBootOrderLib.inf
>>    FileExplorerLib|MdeModulePkg/Library/FileExplorerLib/FileExplorerLib.inf
>>    PciPcdProducerLib|ArmVirtPkg/Library/FdtPciPcdProducerLib/FdtPciPcdProducerLib.inf
>> @@ -348,7 +347,6 @@
>>    #
>>    # Video support
>>    #
>> -  OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
>>    OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
>>    OvmfPkg/PlatformDxe/Platform.inf
>>
>>
>
> (My R-b stands; these are comments for a possible followup patch.)
>
> Please see:
>
> - commit 84a75f70e903 ("OvmfPkg/QemuVideoDxe: enable ARM builds",
> 2015-02-23),
>
> - commit 05a537945872 ("OvmfPkg/QemuVideoDxe: Helper functions for
> unaligned port I/O.", 2017-04-07)?
>
> In my opinion, we should now revert parts of these commits, in one
> followup patch:
>
> - from the first commit, we should revert only the "VALID_ARCHITECTURES"
> comment change (the rest is built upon by the second commit, and should
> be preserved)
>
> - from the second commit, we should revert the addition of [Sources.ARM,
> Sources.AARCH64].
>
> This boils down to removing ARM and AARCH64 references from the
> QemuVideoDxe.inf file. If you agree, could you please submit such a
> followup patch?
>

Sure, but pending the graphical GRUB discussion.