PV Autotranslate guests (are they used at all?)

PV Autotranslate guests (are they used at all?)

[Andrew Cooper]

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.

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?

~Andrew

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Re: PV Autotranslate guests (are they used at all?)

[Konrad Rzeszutek Wilk]

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
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> https://lists.xen.org/xen-devel

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Re: PV Autotranslate guests (are they used at all?)

[Tim Deegan]

At 20:37 +0000 on 07 Dec (1481143049), Andrew Cooper wrote:
> 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 from me!

Tim.

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Re: PV Autotranslate guests (are they used at all?)

[George Dunlap]

> On Dec 8, 2016, at 4:37 AM, Andrew Cooper <andrew.cooper3@citrix.com> 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 used them for my PhD thesis that I finished up in 2006 — that’s probably why the code is there in the first place.  And the only reason I used PV guests instead of HVM guests is that when I started my work in 2004 HVM guests didn’t exist.  If execution replay is ever implemented again, it would probably be with PVH guests.

So no objection from me either.

 -George

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