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Re: [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server()




On 10.12.20 10:38, Paul Durrant wrote:

Hi Paul.

-----Original Message-----
From: Oleksandr <olekstysh@xxxxxxxxx>
Sent: 09 December 2020 20:36
To: paul@xxxxxxx
Cc: 'Jan Beulich' <jbeulich@xxxxxxxx>; 'Oleksandr Tyshchenko' 
<oleksandr_tyshchenko@xxxxxxxx>;
'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; 'Julien Grall' <julien@xxxxxxx>; 
'Volodymyr Babchuk'
<Volodymyr_Babchuk@xxxxxxxx>; 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 
'George Dunlap'
<george.dunlap@xxxxxxxxxx>; 'Ian Jackson' <iwj@xxxxxxxxxxxxxx>; 'Wei Liu' 
<wl@xxxxxxx>; 'Julien Grall'
<julien.grall@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
Subject: Re: [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server()


Hi Paul.


On 30.11.2020 11:31, Oleksandr Tyshchenko wrote:
--- a/xen/include/xen/ioreq.h
+++ b/xen/include/xen/ioreq.h
@@ -55,6 +55,20 @@ struct ioreq_server {
         uint8_t                bufioreq_handling;
     };
     +/*
+ * This should only be used when d == current->domain and it's not
paused,
Is the "not paused" part really relevant here? Besides it being rare
that the current domain would be paused (if so, it's in the process
of having all its vCPU-s scheduled out), does this matter at all?do
any extra actionsdo any extra actions
No, it isn't relevant, I will drop it.


Apart from this the patch looks okay to me, but I'm not sure it
addresses Paul's concerns. Iirc he had suggested to switch back to
a list if doing a swipe over the entire array is too expensive in
this specific case.
We would like to avoid to do any extra actions in
leave_hypervisor_to_guest() if possible.
But not only there, the logic whether we check/set
mapcache_invalidation variable could be avoided if a domain doesn't
use IOREQ server...
Are you OK with this patch (common part of it)?
How much of a performance benefit is this? The array is small to simply 
counting the non-NULL
entries should be pretty quick.
I didn't perform performance measurements on how much this call consumes.
In our system we run three domains. The emulator is in DomD only, so I
would like to avoid to call vcpu_ioreq_handle_completion() for every
Dom0/DomU's vCPUs
if there is no real need to do it.
This is not relevant to the domain that the emulator is running in; it's 
concerning the domains
which the emulator is servicing. How many of those are there?
Err, yes, I wasn't precise when providing an example.
Single emulator is running in DomD and servicing DomU. So with the
helper in place the vcpu_ioreq_handle_completion() gets only called for
DomU vCPUs (as expected).
Without an optimization the vcpu_ioreq_handle_completion() gets called
for _all_ vCPUs, and I see it as an extra action for Dom0, DomD vCPUs.


On Arm vcpu_ioreq_handle_completion()
is called with IRQ enabled, so the call is accompanied with
corresponding irq_enable/irq_disable.
These unneeded actions could be avoided by using this simple one-line
helper...

The helper may be one line but there is more to the patch than that. I still 
think you could just
walk the array in the helper rather than keeping a running occupancy count.

OK, is the implementation below close to what you propose? If yes, I
will update a helper and drop nr_servers variable.

bool domain_has_ioreq_server(const struct domain *d)
{
      const struct ioreq_server *s;
      unsigned int id;

      FOR_EACH_IOREQ_SERVER(d, id, s)
          return true;

      return false;
}
Yes, that's what I had in mind.

   Paul

Thank you for the clarification.


--
Regards,

Oleksandr Tyshchenko




 


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