Re: [Xen-devel] [PATCH] amd-iommu: remove page merging code

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 27 November 2018 13:02
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Brian Woods <brian.woods@xxxxxxx>; Suravee Suthikulpanit
> <suravee.suthikulpanit@xxxxxxx>; xen-devel <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] amd-iommu: remove page merging code
> 
> >>> On 26.11.18 at 18:30, <paul.durrant@xxxxxxxxxx> wrote:
> > The page merging logic makes use of bits 1-8 and bit 63 of a PTE, which
> used
> > to be specified as ignored. However, bits 5 and 6 are now specified as
> > 'accessed' and 'dirty' bits and their use only remains safe as long as
> > the DTE 'Host Access Dirty' bits remain clear. The code is also of
> dubious
> > benefit and was the subject XSA-275.
> >
> > This patch removes the code, freeing up the remaining PTE 'ignored' bits
> > for other potential use and shortening the source by 170 lines.
> 
> No word at all about the performance implications? Do you have
> any plans to re-introduce properly working page recombining
> code? I realize VT-d doesn't have any either (the maintainers at
> some point in the distant past had promised to implement it, but
> I guess that's long been forgotten), but anyway...
> 

I hope to wire through the order parameter to the implementations eventually, 
which is the right way to do things I think. It also means I'll probably need 
to tweak the PV-IOMMU interface to handle an order parameter before I send 
v.next too.

  Paul

> Jan
> 


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