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

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Message-Id: <200905072200.22007.arnd@arndb.de>
Date:	Thu, 7 May 2009 22:00:21 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Gregory Haskins <gregory.haskins@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, Gregory Haskins <ghaskins@...ell.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Thursday 07 May 2009, Gregory Haskins wrote:
> I guess technically mmio can just be a simple access of the page which
> would be problematic to trap locally without a PF.  However it seems
> that most mmio always passes through a ioread()/iowrite() call so this
> is perhaps the hook point.  If we set the stake in the ground that mmios
> that go through some other mechanism like PFs can just hit the "slow
> path" are an acceptable casualty, I think we can make that work.
> 
> Thoughts?

An mmio that goes through a PF is a bug, it's certainly broken on
a number of platforms, so performance should not be an issue there.

Note that are four commonly used interface classes for PIO/MMIO:

1. readl/writel: little-endian MMIO
2. inl/outl: little-endian PIO
3. ioread32/iowrite32: converged little-endian PIO/MMIO
4. __raw_readl/__raw_writel: native-endian MMIO without checks

You don't need to worry about the __raw_* stuff, as this should never
be used in device drivers.

As a simplification, you could mandate that all drivers that want to
use this get converted to the ioread/iowrite class of interfaces and
leave the others slow.

	Arnd <><
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