Re: [Xen-devel] [PATCH] linux-2.6.18/x86: replace order-based range checking of M2P table by linear one

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Jul 25, 2011 at 03:23:02PM +0100, Jan Beulich wrote:
> >>> On 25.07.11 at 16:19, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>> wrote:
> > On Mon, Jul 25, 2011 at 11:05:22AM +0100, Jan Beulich wrote:
> >> The order-based approach is not only less efficient (requiring a shift
> >> and a compare, typical generated code looking like this
> >> 
> >>    mov     eax, [machine_to_phys_order]
> >>    mov     ecx, eax
> >>    shr     ebx, cl
> >>    test    ebx, ebx
> >>    jnz     ...
> >> 
> >> whereas a direct check requires just a compare, like in
> >> 
> >>    cmp     ebx, [machine_to_phys_nr]
> >>    jae     ...
> >> 
> >> ), but also slightly dangerous in the 32-on-64 case - the element
> >> address calculation can wrap if the next power of two boundary is
> >> sufficiently far away from the actual upper limit of the table, and
> >> hence can result in user space addresses being accessed (with it being
> >> unknown what may actually be mapped there).
> > 
> > You wouldn't have a patch for upstream Linux for this?
> 
> I can try to port this over, but it'll take some time until I can get to
> this (certainly not before returning from the summit).

<nods>Absolutly. Will wait.

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