Re: [Xen-devel] [PATCH v13 12/23] x86: refactor psr: L3 CAT: set value: implement write msr flow.

[Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>]

On 17-07-13 13:21:46, Jan Beulich wrote:
> >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 07/13/17 9:34 AM >>>
> >On 17-07-12 23:20:24, Jan Beulich wrote:
> >> >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 07/13/17 5:00 AM >>>
> >> >On 17-07-12 13:37:02, Jan Beulich wrote:
> >> >> >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 07/06/17 4:07 AM >>>
> >> >> >+    if ( socket == cpu_to_socket(smp_processor_id()) )
> >> >> >+        do_write_psr_msrs(&data);
> >> >> >+    else
> >> >> >+    {
> >> >> >+        unsigned int cpu = get_socket_cpu(socket);
> >> >> >+
> >> >> >+        if ( cpu >= nr_cpu_ids )
> >> >> >+            return -ENOTSOCK;
> >> >> >+        on_selected_cpus(cpumask_of(cpu), do_write_psr_msrs, &data, 
> >> >> >1);
> >> >> 
> >> >> How frequent an operation can this be? Considering that the actual MSR 
> >> >> write(s)
> >> >> in the handler is (are) conditional I wonder whether it wouldn't be 
> >> >> worthwhile
> >> >> trying to avoid the IPI altogether, by pre-checking whether any write 
> >> >> actually
> >> >> needs doing.
> >> >> 
> >> >Yes, I think I can check if the value to set is same as 
> >> >'feat->cos_reg_val[cos]'
> >> >before calling IPI.
> >> 
> >> Well, as said - whether it's worth the extra effort depends on whether 
> >> there is
> >> a (reasonable) scenario where this function may be executed frequently.
> >> 
> >This function is executed when 'psr-cat-set' command is executed. I consult
> >the libvirt guy, this command may be executed frequently under some 
> >scenarios.
> >E.g. user may dynamically adjust the cache allocation for VMs according to 
> >CMT
> >result.
> 
> Hmm, that's not something I would call frequent - in the whole invocation of 
> the
> user mode process the IPI will be lost in the noise. "Frequent" would be 
> something
> the kernel does without direct user mode triggering, like on the context 
> switch
> path, in code running from a timer, or some such.
> 
Then, it is not 'Frequent'. This function is only trigger by user. So, I
will keep current codes. Thanks!

> Jan
> 
> 
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