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67 lines
2.6 KiB
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67 lines
2.6 KiB
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Technical FAQ
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#############
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This FAQ is targetted at developers or technical people that want to
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know more about what's going on under the hood.
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.. _ivshmemshared_ram:
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IVSHMEM/Shared RAM
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------------------
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.. _what_exactly_is_the_ivshmem_device:
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What exactly is the IVSHMEM device?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This is a virtual device that maps a segment of shared memory into the
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guest via a BAR (Base Address Register). It also has additional features
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such as interrupt triggering for synchronization however we do not use
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these.
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.. _what_is_the_ivshmem_device_being_used_for:
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What is the IVSHMEM device being used for?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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One might assume that we are simply using the device for the captured
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frames, this, however, is not entirely accurate. Looking Glass also
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needs to capture mouse shape changes (the mouse cursor), and mouse
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movement events and feed these back to the client to render. We need
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this additional information as we actually are rendering the cursor on
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the client-side, independent of the frame capture. This is why when you
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move your cursor around it doesn't affect the UPS, which is only
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counting frame updates.
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.. _why_do_you_need_the_mouse_positional_information:
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Why do you need the mouse positional information?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Windows has no notion of an absolute pointing device unless you are
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using a tablet, which does work, however, if you also want relative
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input for applications/games that require cursor capture, you need a
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relative input device such as a PS/2 mouse.
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The problem is, due to the design of QEMU or the Windows mouse subsystem
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(not sure which), when the VM has both devices attached (which is the
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default for libvirt), mouse click events are always at the last location
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of the absolute positional device (tablet) even if the cursor has been
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moved with the relative input device.
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Because of this bug, we need to always operate in relative mouse input
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mode, and since factors like windows mouse acceleration, or cursor
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movement by a user application may occur in the guest, we need to pass
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this information back so the client can render the cursor in the correct
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location.
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.. _why_does_lg_poll_for_updates_instead_of_using_interrupts:
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Why does LG poll for updates instead of using interrupts?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Initially, we were using interrupts in early designs however it became
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clear that the performance, especially for high update rate mice was
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extremely poor. This may have improved in recent QEMU versions and
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perhaps should be re-evaluated at some point.
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