On 25.09.2008, at 23:27, Joerg Roedel wrote:
On Thu, Sep 25, 2008 at 09:55:27PM +0200, Alexander Graf wrote:
On 25.09.2008, at 20:47, Joerg Roedel wrote:
I had another possible idea for performance improvement here.
Since we
only inject normal interrupts and exceptions (and not NMI and
such) we
can patch clgi to cli and stgi to sti to save these two intercepts
in
the guests vmrun path.
Any objections/problems with this?
How do we know if we're allowed to inject interrupts with V_INTR set?
Usually IF is on and GIF is off when entering the VM in KVM, so we
allow interrupts to arrive even when IF is clear in the guest...
Hmm yes, this is a problem. So this optimization will not work. We
need
other ways to optimize :)
Well it would work for the KVM-in-KVM case, where we know that VMRUN
is always triggered with IF=1 and V_INTR=1. The only case that hack
fails is when we have IF=0 and V_INTR=1. Everything else should work
just fine. And in this case we would simply issue some VMEXITs 0x60,
so no big deal IMHO. It should be worth the tradeoff of making most
VMMs a lot faster.
There should be a compile-option to enable the "correct" behavior
though. If we join that with the VMLOAD and VMSAVE hack there would be
only the VMRUN and DR exits left. That sounds like a really good
improvement where I wouldn't mind to break some specs :-).
Alex
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