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Re: [Xen-devel] [PATCH 1/2] x86/hvm: improve performance of HVMOP_flush_tlbs


  • To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 27 Dec 2019 14:52:17 +0000
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  • Cc: Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
  • Delivery-date: Fri, 27 Dec 2019 14:52:58 +0000
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On 24/12/2019 13:26, Roger Pau Monne wrote:
> There's no need to call paging_update_cr3 unless CR3 trapping is
> enabled, and that's only the case when using shadow paging or when
> requested for introspection purposes, otherwise there's no need to
> pause all the vCPUs of the domain in order to perform the flush.
>
> Check whether CR3 trapping is currently in use in order to decide
> whether the vCPUs should be paused, otherwise just perform the flush.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

I agree that the existing logic poor, but this direction looks to be
even more fragile.

Instead, I think it would be better to follow the EPT invalidation
example; mark all vcpus as needing a tlb flush, and IPI the domain dirty
mask, having the return-to-guest path do the flushing.

This avoids all vcpu pausing/unpausing activities, and the cost of the
flush is incurred by the target vcpu, rather than the vcpu making the
hypercall accumulate the cost for everything, as well as a large amount
of remote VMCS accesses.

It can probably also remove the need for the flush_vcpu() callback which
is going to be expensive due to retpoline, and whose contents are trivial.

~Andrew

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