From: Leif Lindholm <leif.lindholm@linaro.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Laszlo Ersek <lersek@redhat.com>,
"edk2-devel@lists.01.org" <edk2-devel@lists.01.org>,
Jordan Justen <jordan.l.justen@intel.com>
Subject: Re: [PATCH] ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel
Date: Wed, 23 Aug 2017 14:15:16 +0100 [thread overview]
Message-ID: <20170823131516.6f4d2p6jy222ch6m@bivouac.eciton.net> (raw)
In-Reply-To: <CAKv+Gu9jRikAnYMXUz+9d6ZYq0wL+3Rm5mVBns0g+Ai3XxBkdQ@mail.gmail.com>
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.
/
Leif
next prev parent reply other threads:[~2017-08-23 13:12 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-22 16:30 [PATCH] ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel Ard Biesheuvel
2017-08-22 16:47 ` Laszlo Ersek
2017-08-22 16:57 ` Leif Lindholm
2017-08-22 17:16 ` Ard Biesheuvel
2017-08-22 17:38 ` Laszlo Ersek
2017-08-22 19:05 ` Leif Lindholm
2017-08-22 19:14 ` Ard Biesheuvel
2017-08-22 17:02 ` Laszlo Ersek
2017-08-22 17:16 ` Ard Biesheuvel
2017-08-23 13:15 ` Leif Lindholm [this message]
2017-08-23 13:17 ` Ard Biesheuvel
2017-08-23 13:36 ` Laszlo Ersek
2017-08-23 15:00 ` Leif Lindholm
2017-08-24 12:02 ` Leif Lindholm
2017-08-24 12:25 ` Ard Biesheuvel
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