Re: [Xen-devel] PV Autotranslate guests (are they used at all?)

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Dec 07, 2016 at 08:37:29PM +0000, Andrew Cooper wrote:
> Hello,
> 
> While digging around, it looks like there is some major bitrot of the PV
> autotranslate code.
> 
> When constructing an autotranslate domain, tools/libxc/xc_dom_x86.c:
> x86_shadow() sets refcount | translate on the domain.
> 
> The combination of translate != external was excluded by c/s
> 92942fd3d469, which means that PV autotranslate guests can't boot on Xen
> 4.7 or later.
> 
> The shadow emulation code for PV guests (which gets used one way or
> another if any of refcount|translate|external are set) always sets up
> emulation in the same mode as Xen's %cs.  It appears to have had this
> behaviour since its introduction in c/s 1daf5e293b, and presumably means
> that noone has tried running a 32bit autotranslate guest on 64bit Xen in
> anger.
> 
> Does anyone use PV autotranslate guests at all?  I don't believe I have
> never come across one.

I ran it once I thing for fun. Just to see if dom0 could run. But that
was in .. Xen 4.1 days?

> 
> The current shadow code excludes the translate without refcounts case,
> but the converse case (refcounts without translate) doesn't make sense. 
> Without Xen performing translation, there are no shadow tables to
> reference count in the first place.
> 
> This means that the only sensible shadow mode is refcount | translate |
> external, which allows PV emulation code in arch/x86/mm/shadow/ to be
> dropped, as PV guests necessarily can't be external.  Doing so however
> would definitely be the end of autotranslate mode.
> 
> Given that it hasn't booted on the past two releases of Xen, and doesn't
> appear to have ever worked in one common case, does anyone have any
> objection if I remove all vestigial pieces and permanently lay the
> feature to rest in SCM history?

No objection.




> 
> ~Andrew
> 
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