Re: [Xen-devel] [PATCH v4 1/7] x86/xpti: avoid copying L4 page table contents when possible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.18 at 11:06, <jgross@xxxxxxxx> 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 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>



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