Re: [Xen-devel] [PATCH v4 1/8] x86: NOP out XPTI entry/exit code when it's not in use

[Juergen Gross <jgross@xxxxxxxx>]

On 19/03/18 14:37, Jan Beulich wrote:
> Introduce a synthetic feature flag to use alternative instruction
> patching to NOP out all code on entry/exit paths. Having NOPs here is
> generally better than using conditional branches.
> 
> Also change the limit on the number of bytes we can patch in one go to
> that resulting from the encoding in struct alt_instr - there's no point
> reducing it below that limit, and without a check being in place that
> the limit isn't actually exceeded, such an artificial boundary is a
> latent risk.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Just did a parallel make of the hypervisor with and without the patch,
with xpti=true and with xpti=false (values in braces are stddev).

                       elapsed         system          user
unpatched, xpti=false: 118.69 ( 1.40)  168.38 (12.64)  186.11 ( 5.49)
unpatched, xpti=true : 128.02 ( 5.97)  219.66 (23.06)  197.84 ( 4.53)
patched,   xpti=false:  90.65 ( 6.63)   99.50 (14.79)  180.35 ( 5.97)
patched,   xpti=true : 111.69 ( 9.93)  163.63 (13.05)  181.22 ( 3.71)


Juergen

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