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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.05.15 at 13:48, <JBeulich@xxxxxxxx> wrote:
>>>> On 21.05.15 at 13:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>> 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.
> 
> Good idea.

Sadly this can't be a BUILD_BUG_ON(), as STUBS_PER_PAGE isn't a
compile time constant (due to the use of max()). I made it an
ASSERT() for the time being.

Jan


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