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Re: [Xen-devel] [PATCH v3 0/4] x86: accommodate 32-bit PV guests with SMEP/SMAP handling

On Thu, Mar 17, 2016 at 01:50:39AM -0600, Jan Beulich wrote:
> As has been explained previously[1], SMAP (and with less relevance
> also SMEP) is not compatible with 32-bit PV guests which aren't
> aware/prepared to be run with that feature enabled. Andrew's
> original approach either sacrificed architectural correctness for
> making 32-bit guests work again, or disabled SMAP also for not
> insignificant portions of hypervisor code, both by allowing to control
> the workaround mode via command line option.
> This alternative approach disables SMEP and SMAP only while
> running 32-bit PV guest code plus a few hypervisor instructions
> early after entering hypervisor context from or late before exiting
> to such guests. Those few instructions (in assembly source) are of
> course much easier to prove not to perform undue memory
> accesses than code paths reaching deep into C sources.
> The 4th patch really is unrelated except for not applying cleanly
> without the earlier ones, and the potential having been noticed
> while putting together the 2nd one.
> 1: move cached CR4 value to struct cpu_info
> 2: suppress SMEP and SMAP while running 32-bit PV guest code
> 3: use optimal NOPs to fill the SMEP/SMAP placeholders
> 4: use 32-bit loads for 32-bit PV guest state reload
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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