lkml.org 
[lkml]   [2007]   [Aug]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] Add I/O hypercalls for i386 paravirt
Avi Kivity wrote:
> 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. Worse, even with hardware assist such
>> as VT, the exit reason alone is not sufficient to determine the true
>> nature of the faulting instruction, requiring a complex and costly
>> instruction decode and simulation.
>>
>> This patch provides hypercalls for the i386 port I/O instructions,
>> which vastly helps guests which use native-style drivers. For certain
>> VMI workloads, this provides a performance boost of up to 30%. We
>> expect KVM and lguest to be able to achieve similar gains on I/O
>> intensive workloads.
>>
>>
>
>
> Won't these workloads be better off using paravirtualized drivers?
> i.e., do the native drivers with paravirt I/O instructions get anywhere
> near the performance of paravirt drivers?
>

Yes, in general, this is true (better off with paravirt drivers).
However, we have "paravirt" drivers which run in both
fully-paravirtualized and fully traditionally virtualized environments.
As a result, they use native port I/O operations to interact with
virtual hardware.

Since not all hypervisors have paravirtualized driver infrastructures
and guest O/S support yet, these hypercalls can be advantages to a wide
range of scenarios. Using I/O hypercalls as such gives exactly the same
performance as paravirt drivers for us, by eliminating the costly decode
path, and the simplicity of using the same driver code makes this a huge
win in code complexity.

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/

\
 
 \ /
  Last update: 2007-08-22 07:57    [W:0.135 / U:14.728 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site