Re: [Xen-devel] [PATCH v3 1/6] x86: NOP out XPTI entry/exit code when it's not in use

[Juergen Gross <jgross@xxxxxxxx>]

On 15/03/18 20:44, Andrew Cooper wrote:
> On 13/03/18 13:47, 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>
>> Tested-by: Juergen Gross <jgross@xxxxxxxx>
>> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
> 
> I'm afraid that I still have misgivings about this patch.
> 
> While I'm quite willing to trust that it functions correctly, it is
> taking a some code which is almost impossible to follow already, and
> making it substantially more complicated to follow, for what appears to
> be a fractional gain.
> 
> The two distinct areas of concern are the split interrupt re-enablement
> (which really doesn't buy us anything useful), and how obvious the
> nopping is (where in the .Lxcpt_cr3_start case, the ALTERNATIVE_NOP is
> 111 lines (!) away from the code it applies to).
> 
> I.e. I'm struggling to decide whether it falls into the category of
> unnecessary micro-optimisation or not.
> 
> Therefore, I'd like to consider what other XPTI changes we're expecting
> to get, and whether those have an impact.
> 
> I've got a patch (which I've not had time to submit upstream yet, but
> would like to get in for 4.11) which implements a crude "no XPTI for
> dom0" mode.  Performance testing shows that in scenarios running only
> HVM guests (a very common XenServer setup), the difference between dom0
> XPTI-ness can be up to 40% in terms of aggregate network/disk throughput.

That's something my series is covering already.

Right now I'm hunting a bug in my PCID patch (the last of my series)
which seems to relate to the INVPCID adaptions I've made addressing the
comments to V2 of my series.

I can post my series without the last patch if you want. This will give
a speedup of the simple compilation benchmark of about 20% system time
and 12% elapsed time. And it will have the dom0 without XPTI option.


Juergen

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