linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sat, 09 May 2009 08:01:53 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Avi Kivity <avi@...hat.com>, Chris Wright <chrisw@...s-sol.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] generic hypercall support

Anthony Liguori wrote:
> Avi Kivity wrote:
>>
>> Hmm, reminds me of something I thought of a while back.
>>
>> We could implement an 'mmio hypercall' that does mmio reads/writes
>> via a hypercall instead of an mmio operation.  That will speed up
>> mmio for emulated devices (say, e1000).  It's easy to hook into Linux
>> (readl/writel), is pci-friendly, non-x86 friendly, etc.
>
> By the time you get down to userspace for an emulated device, that 2us
> difference between mmio and hypercalls is simply not going to make a
> difference.

I don't care about this path for emulated devices.  I am interested in
in-kernel vbus devices.

>   I'm surprised so much effort is going into this, is there any
> indication that this is even close to a bottleneck in any circumstance?

Yes.  Each 1us of overhead is a 4% regression in something as trivial as
a 25us UDP/ICMP rtt "ping".
>
>
> We have much, much lower hanging fruit to attack.  The basic fact that
> we still copy data multiple times in the networking drivers is clearly
> more significant than a few hundred nanoseconds that should occur less
> than once per packet.
for request-response, this is generally for *every* packet since you
cannot exploit buffering/deferring.

Can you back up your claim that PPC has no difference in performance
with an MMIO exit and a "hypercall" (yes, I understand PPC has no "VT"
like instructions, but clearly there are ways to cause a trap, so
presumably we can measure the difference between a PF exit and something
more explicit).

We need numbers before we can really decide to abandon this
optimization.  If PPC mmio has no penalty over hypercall, I am not sure
the 350ns on x86 is worth this effort (especially if I can shrink this
with some RCU fixes).  Otherwise, the margin is quite a bit larger.

-Greg




Download attachment "signature.asc" of type "application/pgp-signature" (267 bytes)