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@xxxxxxxxxx>
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.
--
error compiling committee.c: too many arguments to function
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