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

[Laszlo Ersek <lersek@redhat.com>]

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?

Thanks!
Laszlo