Re: [Xen-devel] Fetching instructions after page-fault, near page boundary?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 2 Jun 2006, at 18:07, Petersson, Mats wrote:

> > I wouldn't trust it. What if you have code running in paged
> > memory (e.g., random privileged userspace process)? Pages can
> > disappear under your feet. I think you need to remember how
> > many bytes you managed to read and do the job thoroughly.
> > It's not that much extra code.
> But that means that we'd have to parse the instruction bytes in Xen
> (since we can't read them as trivially in QEMU) and figure out how many
> bytes the instruction is. Since both AMD and Intel have problems with
> getting the correct number of bytes from the processor during a
> page-fault intercept, it's no help that Intel SOMETIMES have a correct
> number of bytes in a VMCS entry...
Read as many as you can, up to 15. Tell QEMU how many you actually 
managed to read.
> How do we do it properly, if there's non-present page, re-inject the
> page-fault, I guess?
Just try re-executing the instruction (i.e. directly return to the 
guest). If the page has become unmapped then the processor should 
handle the fault on instruction fetch.
 -- Keir


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