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Re: [PATCH 1/8] IOMMU/x86: drop locking from quarantine_init() hooks
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 12 Apr 2022 15:14:52 +0200
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- Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 12 Apr 2022 13:15:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11.04.2022 12:01, Andrew Cooper wrote:
> On 11/04/2022 10:35, Jan Beulich wrote:
>> Prior extension of these functions to enable per-device quarantine page
>> tables already didn't add more locking there, but merely left in place
>> what had been there before. But really locking is unnecessary here:
>> We're running with pcidevs_lock held (i.e. multiple invocations of the
>> same function [or their teardown equivalents] are impossible, and hence
>> there are no "local" races), while all consuming of the data being
>> populated here can't race anyway due to happening sequentially
>> afterwards. See also the comment in struct arch_pci_dev.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> It is only legitimate to drop these calls if you delete the mapping_lock
> entirely. Otherwise you're breaking the semantics of mapping_lock.
>
> Your argument of "well this is already guarded by the pci lock" means
> that these are uncontended lock/unlock operations, and therefore not
> interesting to drop in the first place.
>
> This patch is specifically setting us up for an XSA in the future when
> the behaviour of the the PCI lock changes, the fix for which will be
> revert this patch.
Further to my earlier reply, may I remind you that changes to the PCI
lock won't go unnoticed here, as there are respective ASSERT()s in
place.
Jan
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