Messages in this thread | | | Date | Wed, 22 Aug 2007 10:07:47 -0700 | From | Zachary Amsden <> | Subject | Re: [PATCH] Add I/O hypercalls for i386 paravirt |
| |
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |