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

[Paolo Bonzini <pbonzini@redhat.com>]

Avi Kivity wrote:
> 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.

Won't the guest do that automatically through DMI?

I think we have four cases:

1) non-buggy chipset, host uses 0x80 and guest cannot crash the machine; 
this is okay.

2) buggy chipset, both host and guest use 0xed thanks to DMI detection; 
and (since 0x80 is not passthrough) the guest cannot crash the machine, 
so this is okay.

3) host is given explicit io_delay=0xed, but the guest isn't.  However, 
0x80 is not passthrough so the guest can use port 0x80 safely even on 
buggy hardware.

4) a buggy chipset is not detected, so io_delay=0x80 and 0x80 is made 
passthrough.  In principle the guest could crash the machine; however 
the host will have written to 0x80 at startup, most likely causing a 
crash way before a guest is started.  So this should not be a problem 
either.

Paolo