Re: [RFC PATCH 0/7] Add support for monitoring guest TLB operations

[Paolo Bonzini]

On 16/08/2016 12:45, Punit Agrawal wrote:
> Hi,
> 
> ARMv8 supports trapping guest TLB maintenance operations to the
> hypervisor. This trapping mechanism can be used to monitor the use of
> guest TLB instructions.
> 
> As taking a trap for every TLB operation can have significant
> overhead, trapping should only be enabled -
> 
> * on user request
> * for the VM of interest
> 
> This patchset adds support to listen to perf trace event state change
> notifications. The notifications and associated context are then used
> to enable trapping of guest TLB operations when requested by the
> user. The trap handling generates trace events (kvm_tlb_invalidate)
> which can already be counted using existing perf trace
> functionality.
> 
> Trapping of guest TLB operations is disabled when not being monitored
> (reducing profiling overhead).
> 
> I would appreciate feedback on the approach to tie the control of TLB
> monitoring with perf trace events (Patch 1) especially if there are
> any suggestions on avoiding (or reducing) the overhead of "perf trace"
> notifications.
> 
> I looked at using regfunc/unregfunc tracepoint hooks but they don't
> include the event context. But the bigger problem was that the
> callbacks are only called on the first instance of simultaneously
> executing perf stat invocations.
> 
> The patchset is based on v4.8-rc2 and adds support for monitoring
> guest TLB operations on 64bit hosts. If the approach taken in the
> patches is acceptable, I'll add 32bit host support as well.
> 
> With this patchset, 'perf' tool when attached to a VM process can be
> used to monitor the TLB operations. E.g., to monitor a VM with process
> id 4166 -
> 
> # perf stat -e "kvm:kvm_tlb_invalidate" -p 4166
> 
> Perform some operations in VM (running 'make -j 7' on the kernel
> sources in this instance). Breaking out of perf shows -
> 
> Performance counter stats for process id '4166':
> 
>          7,471,974      kvm:kvm_tlb_invalidate
> 
>      374.235405282 seconds time elapsed
> 
> All feedback welcome.

Can you explain what this is used for?  In other words, why would this
be used instead of just running perf in the guest?

Thanks,

Paolo