linux-kernel - Re: [RFC] pci: Rework config space blocking services

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Message-ID: <20110906080410.GD13286@redhat.com>
Date:	Tue, 6 Sep 2011 11:04:10 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Brian King <brking@...ibm.com>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	"Hans J. Koch" <hjk@...sjkoch.de>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC] pci: Rework config space blocking services

On Tue, Sep 06, 2011 at 09:18:13AM +0200, Jan Kiszka wrote:
> >> @@ -401,36 +403,58 @@ int pci_vpd_truncate(struct pci_dev *dev, size_t size)
> >>  EXPORT_SYMBOL(pci_vpd_truncate);
> >>
> >>  /**
> >> - * pci_block_user_cfg_access - Block userspace PCI config reads/writes
> >> + * pci_block_cfg_access - Block PCI config reads/writes
> > 
> > This comment seems confusing. We don't in fact block all config
> > reads writes. Instead we block userspace accesses and
> > concurrent block requests.
> 
> I'm open for a better suggestion that summarize the more verbose (and
> hopefully clearer) explanation below.

I think the problem is, it doesn't block config access
and we call it pci_block_cfg_access.

Thinking about it, doesn't this behave somewhat like a lock?
How about

pci_user_cfg_access_trylock
pci_user_cfg_access_lock
pci_user_cfg_access_unlock

And then:
 * pci_user_cfg_access_lock - Lock userspace PCI config access
 *
 * When locked, any userspace reads or writes to config space
 * and concurrent lock requests will sleep, and trylock requests
 * will fail, until pci_user_cfg_access_unlock is called.

I had a brief thought of using an rwsem internally, but
this would make trylock fail if userspace does config read,
changing semantics.

-- 
MST
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