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 03/04/18 19:48, Juergen Gross wrote:
> 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).

The unpatched version was configured differently than the patched one.
So the real numbers are:

                       elapsed         system          user
unpatched, xpti=false:  89.96 ( 8.07)   97.05 ( 5.69)  178.64 ( 2.39)
unpatched, xpti=true : 113.42 ( 9.80)  165.99 (15.10)  180.99 ( 2.66)
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)

So the XPTI case is a little bit faster with the patch, while the
non-XPTI case is a little bit slower.

Juergen

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