Re: [Xen-devel] [PATCH] x86: slightly reduce Meltdown band-aid overhead

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 01/18/2018 03:39 PM, Jan Beulich wrote:
> I'm not sure why I didn't do this right away: By avoiding to make any
> of the cloned directmap PTEs global, there's no need to fiddle with
> CR4.PGE on any of the entry paths. Only the exit paths need to flush
> global mappings.
> 
> The reduced flushing, however, implies that we now need to have
> interrupts off on all entry paths until after the page table switch, so
> that flush IPIs can't arrive with the restricted page tables still
> active, but only a non-global flush happening with the CR3 loads. Along
> those lines the "sync" IPI after L4 entry updates now needs to become a
> real (and global) flush IPI, so that inside Xen we'll also pick up such
> changes.
> 
> Take the opportunity and also do a GET_CURRENT() -> __GET_CURRENT()
> transition the original patch missed.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

FWIW, for me this patch reduces the overhead from 38% to 28%.  Below are
times from a "time make -j 4 xen"

* xpti=off: 63s
* [xpti default]: 87s (+38%)
* + this patch: 81s (+28%)

 -George

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