Re: [PATCH] pci: a workaround for nonstandard PCI devices whose PBA shares

From: George Dunlap <george.dunlap@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>,
	"Chao Gao" <chao.gao@intel.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Tim Deegan <tim@xen.org>, Ian Jackson <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xen.org, Julien Grall <julien.grall@arm.com>,
	Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH] pci: a workaround for nonstandard PCI devices whose PBA shares
Date: Thu, 5 Apr 2018 10:40:37 +0100	[thread overview]
Message-ID: <f53b84af-048c-ab6e-edae-c63134174984@citrix.com> (raw)
In-Reply-To: <20180405093439.2utffzwxnpnf72ji@MacBook-Pro-de-Roger.local>

On 04/05/2018 10:34 AM, Roger Pau Monné wrote:
> On Wed, Apr 04, 2018 at 11:29:39PM +0800, Chao Gao wrote:
>> ... the same page with other registers which are not relevant to MSI-X. Xen
>> marks pages where PBA resides as read-only. When assigning such devices to
>> guest, device driver writes MSI-X irrelevant registers on those pages would
>> lead to an EPT violation and the guest is destroyed because no handler is
>> registered for those address range. In order to make guest capable to use such
>> kind of devices, trapping very frequent write accesses is not a good idea for
>> it would significantly impact the performance.
>>
>> This patch provides a workaround with caveat. Specifically, an option is
>> introduced to specify a list of devices. For those devices, Xen doesn't
>> control the access right to pages where PBA resides. Hence, guest device
>> driver is able to write those pages and functions well. Note that adding an
>> untrusted device to this option may endanger security of the entire system.
>>
>> Signed-off-by: Chao Gao <chao.gao@intel.com>
>> ---
>>  docs/misc/xen-command-line.markdown | 10 +++++++++
>>  xen/arch/x86/msi.c                  |  7 ++++--
>>  xen/drivers/passthrough/pci.c       | 45 +++++++++++++++++++++++++++++++++++--
>>  xen/include/asm-x86/msi.h           |  1 +
>>  4 files changed, 59 insertions(+), 4 deletions(-)
>>
>> diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
>> index b353352..e382513 100644
>> --- a/docs/misc/xen-command-line.markdown
>> +++ b/docs/misc/xen-command-line.markdown
>> @@ -1423,6 +1423,16 @@ Defaults to booting secondary processors.
>>  
>>  > Default: `on`
>>  
>> +### pba\_quirk
> 
> pba_write_allowed would be better, pba_quirk is too generic IMO.

'quirk' was I think requested by Jan; and my understanding is that the
word clearly indicates that the behavior in question is a workaround for
hardware which is not compliant with the appropriate specification.  If
you grep the source tree for 'quirk' you'll find a fairly large number.

pba_shared_quirk might be slightly more descriptive.

 -George

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