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Re: [Xen-devel] [PATCH v2 1/4] x86: move syscall trampolines off the stack

On 21/05/15 11:15, Jan Beulich wrote:
> This is needed as stacks are going to become non-executable. Use
> separate stub pages (shared among suitable CPUs on the same node)
> instead.
> Stub areas (currently 128 bytes each) are being split into two parts -
> a fixed usage one (the syscall ones) and dynamically usable space,
> which will be used by subsequent changes to hold dynamically generated
> code during instruction eumlation.
> While sharing physical pages among certain CPUs on the same node, for
> now the virtual mappings get established in distinct pages for each
> CPU. This isn't a strict requirement, but simplifies VA space
> management for this initial implementation: Sharing VA space would
> require additional tracking of which areas are currently in use. If
> the VA and/or TLB overhead turned out to be a problem, such extra code
> could easily be added.
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

It might be wise to have a BUILD_BUG_ON() which confirms that
STUBS_PER_PAGE is a power of two, which is a requirement given the way
it is used.

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