Re: [Xen-devel] [PATCH v9 1/9] x86/xpti: avoid copying L4 page table contents when possible

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 13:33 +0200 on 26 Apr (1524749590), Juergen Gross wrote:
> For mitigation of Meltdown the current L4 page table is copied to the
> cpu local root page table each time a 64 bit pv guest is entered.
> 
> Copying can be avoided in cases where the guest L4 page table hasn't
> been modified while running the hypervisor, e.g. when handling
> interrupts or any hypercall not modifying the L4 page table or %cr3.
> 
> So add a per-cpu flag indicating whether the copying should be
> performed and set that flag only when loading a new %cr3 or modifying
> the L4 page table.  This includes synchronization of the cpu local
> root page table with other cpus, so add a special synchronization flag
> for that case.
> 
> A simple performance check (compiling the hypervisor via "make -j 4")
> in dom0 with 4 vcpus shows a significant improvement:
> 
> - real time drops from 112 seconds to 103 seconds
> - system time drops from 142 seconds to 131 seconds
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Tim Deegan <tim@xxxxxxx>

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