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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.12.2019 13:10, Roger Pau Monné wrote:
> On Fri, Dec 27, 2019 at 02:52:17PM +0000, Andrew Cooper wrote:
>> 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.
> 
> AFAICT there's no need to call the tlb flush, the vmexit/vmentry
> itself will perform the necessary flushes, so the only requirement is
> to IPI the pCPUs in order to force a vmexit.

TLB flushing is at best conditional upon VM entry - see the callers
of hvm_asid_handle_vmenter().

Jan

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