[Xen-devel] Re: invalid PTE for xen_start-info ?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 3/10/06 7:06 pm, "Rik van Riel" <riel@xxxxxxxxxx> wrote:

> pte 0000000019800027 (real 0010000035010027)
> 
> That last PTE value does not look like a valid x86-64 PTE
> value to me.  That high bit is not the NX bit, nor is it
> within the physical address range of the system in question.
> 
> What's going on here?
> 
> Is this a bug you would like a fix for?

It's not a bug -- it's one of the available-for-software flags that is
stolen by Xen to indicate a kernel PTE. This is done so that we can
distinguish kernel and user mappings, so that the latter can have the global
bit set. Sounds weird, but it avoids flushing user mappings from the TLB
when executing syscalls (we have to change %cr3 value when switching between
guest-user and guest-kernel modes).

 -- Keir



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