Xen devel - Sep 2009 - [PATCH xen-unstable] Remove PSE flag from guest CR4

When I originally did the Xen hypervisor hugepage patch I also allowed guests 
to see the PSE flag in CR4.  This has proven to be a mistake.  Here is a patch 
to remove that functionality.

All Linux kernels will still allow userspace hugepages without the flag.

Dave McCracken


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If you hide PSE in CR4 and CPUID, how does the guest know it can use
superpages?

 -- Keir

On 14/09/2009 18:27, "Dave McCracken" <dcm@mccr.org> wrote:
> 
> When I originally did the Xen hypervisor hugepage patch I also allowed
guests
> to see the PSE flag in CR4.  This has proven to be a mistake.  Here is a
patch
> to remove that functionality.
> 
> All Linux kernels will still allow userspace hugepages without the flag.
> 
> Dave McCracken


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On Monday 14 September 2009, Keir Fraser wrote:
> If you hide PSE in CR4 and CPUID, how does the guest know it can use
> superpages?
Linux assumes it will just work.

Since hugepages in Linux are an optional feature the kernel will boot even on 
a machine that doesn''t support PSE.  Applications that use hugepages
will get
a nasty error.

With the PSE bit set in CPUID Linux tries to do things like map the entire 
kernel in hugepages.

Dave McCracken

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On 09/14/09 10:53, Dave McCracken wrote:
> Linux assumes it will just work.
>
> Since hugepages in Linux are an optional feature the kernel will boot even
on
> a machine that doesn''t support PSE.  Applications that use
hugepages will get
> a nasty error.
>   
That sounds like a Linux bug; at the very least, it sounds very fragile
to rely on this behaviour.

    J

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On Monday 14 September 2009, Jeremy Fitzhardinge wrote:
> On 09/14/09 10:53, Dave McCracken wrote:
> > Linux assumes it will just work.
> >
> > Since hugepages in Linux are an optional feature the kernel will boot
> > even on a machine that doesn''t support PSE.  Applications
that use
> > hugepages will get a nasty error.
> 
> That sounds like a Linux bug; at the very least, it sounds very fragile
> to rely on this behaviour.
Perhaps, but hugepages has always worked like that.  I think the reality is 
that any machine small enough/old enough to not support PSE is way too small 
for hugepages to be useful on it, so no one tries to run it on those machines.

Dave McCracken

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