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Re: [Xen-devel] [PATCH v3 7/7] x86/tlb: use Xen L0 assisted TLB flush when available


  • To: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 28 Jan 2020 17:20:25 +0000
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  • Delivery-date: Tue, 28 Jan 2020 17:20:32 +0000
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On 28/01/2020 17:16, Roger Pau Monné wrote:
>>>> OOI why isn't tlb_clk_enabled set to false when Xen determines to use L0
>>>> assisted flush?
>>> L0 assisted flush can fail (ie: return an error), and in that case Xen
>>> would be better to continue using the timestamped tlb, as it could
>>> avoid some flushes.
>> Do you need to set tlb_clk_enabled in that case?
> AFAICT it's safe to enable the TLB timestamps after being disabled,
> but hypervisor_flush_tlb could fail intermittently with EBUSY in the
> Xen implementation, and I don't really want that to cause spurious
> enabling of the timestamps periodically.

What causes -EBUSY?

I don't think it is reasonable for the hypercall to fail like that. 
There is no circumstance ever where a TLB flush is wanted, and it is
able to be deferred.

Forcing all callers to loop while busy is a terrible interface.

~Andrew

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