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

[Avi Kivity <avi@redhat.com>]

On 06/22/2009 01:11 PM, Paolo Bonzini wrote:
> 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?

As Gleb said, the guest has a virtual DMI table.

>
> 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.

This is all true, but pass through is slower (at least on some chipsets) 
than in-kernel emulation, so I'd rather stay away from it.

-- 
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