Re: [Xen-users] [Xen VGA Passthrough] DomU performance inversely proportional to VM memory size when VGA passthrough is enabled

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Sun, 2014-09-14 at 21:40 +0200, Andrej Komelj wrote:
> Does anybody know what else I could try/check?

This sounds pretty odd to me. As a wild stab in the dark I wonder if
enabling passthrough means that some operation now has to be trapped and
handled in the hypervisor in a way which requires walking over the
entire (populated) address space, that would be an operation which was
proportional to the RAM size and if it was happening often could explain
what you are seeing.

I'm not really up to date on x86 HVM stuff, but for example I think
passthrough vs. none makes some difference to the default cacheability
attributes used for RAM in the second stage page tables, and there are
some cache enable/disable operations which then get more/less expensive
due to that (I'm really fuzzy on the details as you might be able to
tell!).

From your logs it looks like you are running with HAP enabled. It might
be worth trying without (i.e. using shadow mode) just to see if it is
hap specific or not. (I think hap=0 in your guest cfg does this)

After that you may want to post to the devel list and see if anyone has
any ideas.

Ian.


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