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



Hi Oleksandr and Paul,

Sorry for jumping late in the conversation.

On 09/12/2020 09:01, Paul Durrant wrote:
-----Original Message-----
From: Oleksandr <olekstysh@xxxxxxxxx>
Sent: 08 December 2020 20:17
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()


On 08.12.20 21:43, Paul Durrant wrote:

Hi Paul

-----Original Message-----
From: Oleksandr <olekstysh@xxxxxxxxx>
Sent: 08 December 2020 16:57
To: Paul Durrant <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 08.12.20 17:33, Oleksandr wrote:
On 08.12.20 17:11, Jan Beulich wrote:

Hi Jan

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?

AFAICT, the maximum number of IOREQ servers is 8 today.


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.

Right, the concern here is this function will be called in an hotpath (everytime we are re-entering to the guest). At the difference of x86, the entry/exit code is really small, so any additional code will have an impact on the overall performance.

That said, the IOREQ code is a tech preview for Arm. So I would be fine going with Paul's approach until we have a better understanding on the performance of virtio/IOREQ.

I am going to throw some more thoughts about the optimization here. The patch is focusing on performance impact when IOREQ is built-in and not used. I think we can do further optimization (which may superseed this one).

get_pending_vcpu() (called from handle_hvm_io_completion()) is overly expensive in particular if you have no I/O forwarded to an IOREQ server. Entry to the hypervisor can happen for many reasons (interrupts, system registers emulation, I/O emulation...) and the I/O forwarded should be a small subset.

Ideally, handle_hvm_io_completion() should be a NOP (at max a few instructions) if there are nothing to do. Maybe we want to introduce a per-vCPU flag indicating if an I/O has been forwarded to an IOREQ server.

This would also us to bypass most of the function if there is nothing to do.

Any thoughts?

In any case this is more a forward looking rather than a request for the current series. What matters to me is we have a functional (not necessarily optimized) version of IOREQ in Xen 4.15. This would be a great step towards using Virto on Xen.

Cheers,

--
Julien Grall



 


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