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

[Oleksandr <olekstysh@gmail.com>]

On 09.12.20 20:58, Julien Grall wrote:
> Hi Oleksandr and Paul,

Hi Julien, Paul.


>
> Sorry for jumping late in the conversation.
>
> On 09/12/2020 09:01, Paul Durrant wrote:
>>> -----Original Message-----
>>> From: Oleksandr <olekstysh@gmail.com>
>>> Sent: 08 December 2020 20:17
>>> To: paul@xen.org
>>> Cc: 'Jan Beulich' <jbeulich@suse.com>; 'Oleksandr Tyshchenko' 
>>> <oleksandr_tyshchenko@epam.com>;
>>> 'Stefano Stabellini' <sstabellini@kernel.org>; 'Julien Grall' 
>>> <julien@xen.org>; 'Volodymyr Babchuk'
>>> <Volodymyr_Babchuk@epam.com>; 'Andrew Cooper' 
>>> <andrew.cooper3@citrix.com>; 'George Dunlap'
>>> <george.dunlap@citrix.com>; 'Ian Jackson' <iwj@xenproject.org>; 'Wei 
>>> Liu' <wl@xen.org>; 'Julien Grall'
>>> <julien.grall@arm.com>; xen-devel@lists.xenproject.org
>>> 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@gmail.com>
>>>>> Sent: 08 December 2020 16:57
>>>>> To: Paul Durrant <paul@xen.org>
>>>>> Cc: Jan Beulich <jbeulich@suse.com>; Oleksandr Tyshchenko 
>>>>> <oleksandr_tyshchenko@epam.com>; Stefano
>>>>> Stabellini <sstabellini@kernel.org>; Julien Grall 
>>>>> <julien@xen.org>; Volodymyr Babchuk
>>>>> <Volodymyr_Babchuk@epam.com>; Andrew Cooper 
>>>>> <andrew.cooper3@citrix.com>; George Dunlap
>>>>> <george.dunlap@citrix.com>; Ian Jackson <iwj@xenproject.org>; Wei 
>>>>> Liu <wl@xen.org>; Julien Grall
>>>>> <julien.grall@arm.com>; xen-devel@lists.xenproject.org
>>>>> 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.
+1


>
>
> 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 fine with Paul's approach for now (I only need a confirmation that 
I got it correctly).


>
>
> 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.
It is true, what I would to add here is the helper also avoids 
unnecessary vcpu_ioreq_handle_completion() calls as well another 
unnecessary action
(mapcache handling logic, although it is not a hotpath) in subsequent 
patch when IOREQ is 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.

Completely agree, current series is quite big) and if we will try to 
make it perfect I am afraid, we won't have it even in Xen 4.16). As for 
proposed optimization - I think it worth considering, I will mention 
about it in the cover letter for the series among other possible things 
such as buffered request, etc.


>
>
> Cheers,
>
-- 
Regards,

Oleksandr Tyshchenko