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

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

On 23 August 2017 at 14:15, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> On Tue, Aug 22, 2017 at 06:16:56PM +0100, Ard Biesheuvel wrote:
>> 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.
>
> So, after looking at the GRUB code, I am leaning towards agreeing that
> this is actually not a problem at all ... probably. The efi_gop driver
> does a Blt() of the entire screen from an off-screen buffer for all
> updates _unless_ it fails to allocate that off-screen buffer.
>
> So, basically, if you run out of memory at that point, it will try to
> preserve a way to get messages out about that. I will send a question
> out to grub-devel regarding this behaviour.
>
> However, looking at the specification, a question remains over how
> software can determine whether direct FB access is possible. I mean, a
> value of 0 seems like a decent hint, but the spec says nothing on the
> topic.
>

It will assume the FB is accessible unless the pixel format is PixelBltOnly