Re: [PATCH] KVM: SVM: Pass through the host kernel's IO delay port

[Avi Kivity <avi@redhat.com>]

On 06/19/2009 01:15 PM, Paolo Bonzini wrote:
> From: Paolo Bonzini<pbonzini@redhat.com>
>
> KVM's optimization of guest port 80 accesses was removed last May 11
> in commit 99f85a.  However, this probably has speed penalties.  I don't
> have a machine to test but the equivalent VMX patch (fdef3ad) reported
> a speedup of 3-5%, and on the Xen mailing list it was mentioned that on
> Xen passing port 80 through had positive effects on startup speed.
>
> We can enable passthrough to the same port the host kernel uses instead.
>    

Since we don't tell the guest to use 0xed, this won't help.  If we can 
tell the guest, we can just tell it not to use any port at all, since 
this is a virtualized environment and there's no buggy hardware to please.

We could speed it up by trapping and emulating the port write in the 
kernel instead of letting userspace handle it (should be faster than 
passthrough on newer hosts).  I'd like to make is configurable from 
userspace though, the kernel has no business trapping random ports.

-- 
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