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

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Date:	Mon, 11 May 2009 15:01:40 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Saturday 09 May 2009, Benjamin Herrenschmidt wrote:
> This was shot down by a vast majority of people, with the outcome being
> an agreement that for IORESOURCE_MEM, pci_iomap and friends must return
> something that is strictly interchangeable with what ioremap would have
> returned.
> 
> That means that readl and writel must work on the output of pci_iomap()
> and similar, but I don't see why __raw_writel would be excluded there, I
> think it's in there too.

One of the ideas was to change pci_iomap to return a special token
in case of virtual devices that causes iowrite32() to do an hcall,
and to just define writel() to do iowrite32().

Unfortunately, there is no __raw_iowrite32(), although I guess we
could add this generically if necessary.

> Direct dereference is illegal in all cases though.

right.
 
> The token returned by pci_iomap for other type of resources (IO for
> example) is also only supported for use by iomap access functions
> (ioreadXX/iowriteXX) , and IO ports cannot be passed directly to those
> neither.

That still leaves the option to let drivers pass the IORESOURCE_PVIO
for its own resources under some conditions, meaning that we will
only use hcalls for I/O on these drivers but not on others, as Chris
explained earlier.

	Arnd <><
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