Re: [PATCH] Add I/O hypercalls for i386 paravirt

Zachary Amsden
2007-08-22T17:13:09Z

Andi Kleen wrote:
> On Wed, Aug 22, 2007 at 09:48:25AM -0700, Zachary Amsden wrote:
>
>> Andi Kleen wrote:
>>
>>> On Tue, Aug 21, 2007 at 10:23:14PM -0700, Zachary Amsden wrote:
>>>
>>>
>>>> In general, I/O in a virtual guest is subject to performance problems.
>>>> The I/O can not be completed physically, but must be virtualized. This
>>>> means trapping and decoding port I/O instructions from the guest OS.
>>>> Not only is the trap for a #GP heavyweight, both in the processor and
>>>> the hypervisor (which usually has a complex #GP path), but this forces
>>>> the hypervisor to decode the individual instruction which has faulted.
>>>>
>>>>
>>> Is that really that expensive? Hard to imagine.
>>>
>>>
>> You have an expensive (16x cost of hypercall on some processors)
>>
>
> Where is the difference comming from? Are you using SYSENTER
> for the hypercall? I can't really see you using SYSENTER,
> because how would you do system calls then? I bet system calls
> are more frequent than in/out, so if you have decide between the
> two using them for syscalls is likely faster.
>

We use sysenter for hypercalls and also for system calls. :)

> Also I fail to see the fundamental speed difference between
>
> mov index,register
> int 0x...
> ...
> switch (register)
> case xxxx: do emulation
>

Int (on p4 == ~680 cycles).

> versus
>
> out ...
> #gp
> -> switch (*eip) {
> case 0xee: /* etc. */
> do emulation
>

GP = ~2000 cycles.

>> to verify protection in the page tables mapping the page allows
>> execution (P, !NX, and U/S check). This is a lot more expensive than a
>>
>
> When the page is not executable or not present you get #PF not #GP.
> So the hardware already checks that.
>
> The only case where you would need to check yourself is if you emulate
> NX on non NX capable hardware, but I can't see you doing that.
>

No, it doesn't. Between the #GP and decode, you have an SMP race where
another processor can rewrite the instruction.

Zach