Re: [Xen-devel] [PATCH v2 0/2] AMD/IOMMU: re-work mode updating

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.11.2019 18:29,  Durrant, Paul  wrote:
>> -----Original Message-----
>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Jan
>> Beulich
>> Sent: 14 November 2019 16:42
>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Juergen Gross <jgross@xxxxxxxx>; Sander Eikelenboom
>> <linux@xxxxxxxxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Subject: [Xen-devel] [PATCH v2 0/2] AMD/IOMMU: re-work mode updating
>>
>> update_paging_mode() in the AMD IOMMU code expects to be invoked with
>> the PCI devices lock held. The check occurring only when the mode
>> actually needs updating, the violation of this rule by the majority
>> of callers did go unnoticed until per-domain IOMMU setup was changed
>> to do away with on-demand creation of IOMMU page tables.
> 
> Wouldn't it be safer to just get rid of update_paging_mode() and start
> with a reasonable number of levels?

Andrew did basically ask the same, but I continue to be unconvinced:
We can't pick a "reasonable" level, we have to pick the maximum a
guest may end up using. Yet why would we want to have all guests pay
the price of at least one unnecessary page walk level? I don't mean
to say I'm entirely opposed, but trading code simplicity for
performance is almost never an easy or obvious decision.

Jan

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