Re: re-writing on powerpc

[Avi Kivity <avi@redhat.com>]

On 12/15/2010 01:16 PM, Sethi Varun-B16395 wrote:
>
> >  -----Original Message-----
> >  From: kvm-ppc-owner@vger.kernel.org [mailto:kvm-ppc-
> >  owner@vger.kernel.org] On Behalf Of Avi Kivity
> >  Sent: Tuesday, December 14, 2010 9:18 PM
> >  To: Yoder Stuart-B08248
> >  Cc: Hollis Blanchard; Alexander Graf; kvm-ppc@vger.kernel.org
> >  Subject: Re: re-writing on powerpc
> >
> >  On 12/14/2010 05:45 PM, Yoder Stuart-B08248 wrote:
> >  >  >   -----Original Message-----
> >  >  >   From: Avi Kivity [mailto:avi@redhat.com]
> >  >  >   Sent: Tuesday, December 14, 2010 2:49 AM
> >  >  >   To: Hollis Blanchard
> >  >  >   Cc: Yoder Stuart-B08248; Alexander Graf; kvm-ppc@vger.kernel.org
> >  >  >   Subject: Re: re-writing on powerpc
> >  >  >
> >  >  >   On 12/13/2010 07:17 PM, Hollis Blanchard wrote:
> >  >  >   >>   Rewriting is dangerous if the guest is unaware of it.  As soon
> >  >  >  as
> >  >  it
> >  >  >   >>   is made aware of it, it might as well actually do it in the
> >  >  >  best
> >  >  way
> >  >  >   >>   that suits it.
> >  >  >   >
> >  >  >   >   Can you list some examples of dangerous scenarios?
> >  >  >   >
> >  >  >
> >  >  >   - guest checksums own kernel pages
> >  >  >   - clever compiler reuses code for constant pool
> >  >  >   - guest patches itself (a la linux alternatives), surprised when it
> >  >  sees a
> >  >  >   different instruction
> >  >  >   - guest jits own kernel code (like Singularity), gets confused when
> >  >  >  it  reads back something it didn't write
> >  >
> >  >  One possible solution to hiding rewriting from guest if it must be
> >  >  hidden is to mark patched pages as execute only.  If a guest reads a
> >  >  patched page, the hypervisor can fix up the read.
> >  >
> >
> >  Yes.  Something that is common to all the problems above is "using code
> >  as data".
> >
> >  However, execute only would only affect the page's mapping, not the page
> >  itself, yes?  So if the page has another mapping, this doesn't work.
> >
>
> But KVM would be aware of guest page mappings, so access permissions for any particular mapping
> can be controlled by KVM.

kvm isn't aware of all guest mappings (only those that were instantiated 
in shadow tlb/pagetables).

-- 
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