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

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 15 November 2019 09:29
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; Sander Eikelenboom <linux@xxxxxxxxxxxxxx>;
> Juergen Gross <jgross@xxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2 0/2] AMD/IOMMU: re-work mode updating
> 
> 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.

I think in this case, versus the hoops your patches have to jump through just 
to save (possibly) a level of IOMMU page walk, the simplicity argument is quite 
compelling... particularly at this stage in the release cycle.
The fact that we don't know, at start of day, what the max gfn of the guest is 
going to be is also something that really ought to be fixed too... but that is 
another debate.

  Paul

> 
> Jan
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