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Re: [Xen-users] Difference between primary and secondary VGA pass through

  • To: xen-users@xxxxxxxxxxxxx
  • From: Gordan Bobic <gordan@xxxxxxxxxx>
  • Date: Fri, 02 May 2014 09:54:06 +0100
  • Delivery-date: Fri, 02 May 2014 08:55:18 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On 2014-05-02 08:08, H. Sieger wrote:
Well, I've recently gave KVM a try and saw primary passthrough working
in my setup - that was until I installed the AMD driver in Windows
after which the guest didn't boot anymore/blue screen .

I believe primary passthrough can shorten the boot time of the guest,
but other than that I wouldn't break my head over that.

Primary passthrough re-POSTs and initializes the GPU via the BIOS.
That means you get to see the BIOS POST and OS loading screen on
the physical rather than emulated GPU. Other than that there is no

The reason why primary passthrough was of interest recently was
because ATI cards/drivers didn't handle rebooting domU gracefully,
and re-POST-ing the card via the BIOS was a way of getting card
back into a usable state without be driver choking on it in various
ways. Now that the slot-level reset is being implemented, it makes
for a simpler, more generic way to reset the card, which I guess
takes some of the pressure for the need for primary passthrough
for ATI cards.

Nvidia binary drivers handle the card initialization gracefully
(provided the card's device ID is in the expensive
Quadro/Tesla/Grid white-list - hence why a lot of us have been
modifying various GeForce cards into equivalent Quadros) whatever
state it was left in, so this was never an issue for those.


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