On Di, 2014-08-26 at 10:22 +0200, Laszlo Ersek wrote:
> When setting up the list of GOP modes offered on QEMU's stdvga ("VGA")
> and
> QXL ("qxl-vga") video devices, QemuVideoBochsModeSetup() filters those
> modes against the available framebuffer size. (Refer to SVN r15288 /
> git
> commit ec88061e.)
>
> The VBE_DISPI_INDEX_VIDEO_MEMORY_64K register of both stdvga and QXL
> is
> supposed to report the size of the drawable, VGA-compatibility
> framebuffer. Instead, up to and including qemu-2.1, this register
> actually
> reports the full video RAM (PCI BAR 0) size.
>
> In case of stdvga, this happens to be correct, because on that card
> the
> full PCI BAR 0 is usable for drawing; there is no difference between
> "drawable framebuffer size" and "video RAM (PCI BAR 0) size".
>
> However, on the QXL card, only an initial portion of the video RAM is
> suitable for drawing, as compatibility framebuffer; and the value
> currently reported by VBE_DISPI_INDEX_VIDEO_MEMORY_64K overshoots the
> valid size. Beyond the drawable range, the video RAM contains buffers
> and
> structures for the QXL guest-host protocol.
>
> Luckily, the size of the drawable QXL framebuffer can also be read
> from a
> register in the QXL ROM BAR (PCI BAR 2), so let's retrieve it from
> there.
>
> Without this fix, OVMF offers too large resolutions on the QXL card
> (up to
> the full size of the video RAM). If a GOP client selects such a
> resolution
> and draws into the video RAM past the compatibility segment, then the
> guest corrupts its communication structures (which is invalid guest
> behavior).
>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Laszlo Ersek <ler...@redhat.com>
Reviewed-by: Gerd Hoffmann <kra...@redhat.com>
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