Re: [PATCH v2 0/2] Introduce runstate area registration with phys address

[Andrii Anisov <andrii.anisov@gmail.com>]

Hello Julien,

On 08.05.19 16:59, Julien Grall wrote:
> Hi,
> 
> On 23/04/2019 09:10, Andrii Anisov wrote:
>> From: Andrii Anisov <andrii_anisov@epam.com>
>>
>> Following discussion [1] it is introduced and implemented a runstate
>> registration interface which uses guest's phys address instead of a virtual one.
>> The new hypercall employes the same data structures as a predecessor, but
>> expects the vcpu_runstate_info structure to not cross a page boundary.
>> The interface is implemented in a way vcpu_runstate_info structure is mapped to
>> the hypervisor on the hypercall processing and is directly accessed during its
>> updates. This runstate area mapping follows vcpu_info structure registration.
>>
>> Permanent mapping of runstate area would consume vmap area on arm32 what is
>> limited to 1G. Though it is assumed that ARM32 does not target the server market
>> and the rest of possible applications will not host a huge number of VCPUs to
>> render the limitation into the issue.
> 
> I am afraid I can't possible back this assumption. As I pointed out in the previous version, I would be OK with the always map solution on Arm32 (pending performance) because it would be possible to increase the virtual address area by reworking the address space.

I'm sorry, I'm not sure what should be my actions about that.

>>
>> The series is tested for ARM64. Build tested for x86. I'd appreciate if someone
>> could check it with x86.
>> The Linux kernel patch is here [2]. Though it is for 4.14.
> 
> The patch looks wrong to me. You are using virt_to_phys() on a percpu area. 
> What does actually promise you the physical address will always be the same?

Sorry for my ignorance here, could you please elaborate more about what is wrong here?


> Are you saying that the command dd is the CPUBurn? I am not sure how this could be considered as a CPUBurn. IHMO, this is more IO related.

Both /dev/null and /dev/zero are virtual devices no actual IO is performed during their operations, all the load is CPU (user and sys).

> 
>>        VCPU(dX)->idle->VCPU(dX).
>>        with following results:
>>
>>                              mapped               mapped
>>                              on access            on init
>>        GLMark2 320x240       2852                 2877          +0.8%
>>            +Dom0 CPUBurn     2088                 2094          +0.2%
>>        GLMark2 800x600       2368                 2375          +0.3%
>>            +Dom0 CPUBurn     1868                 1921          +2.8%
>>        GLMark2 1920x1080     931                  931            0%
>>            +Dom0 CPUBurn     892                  894           +0.2%
>>
>>        Please note that "mapped on access" means using the old runstate
>>        registering interface. And runstate update in this case still often fails
>>        to map runstate area like [5], despite the fact that our Linux kernel
>>        does not have KPTI enabled. So runstate area update, in this case, is
>>        really shortened.
> 
> We know that the old interface is broken, so telling us the new interface is faster is not entirely useful. What I am more interested is how it if you use a guest physical address on the version "Mapped on access".

Hm, I see your point. Well, I can make it for ARM to compare performance.

> 
>>
>>
>>        Also it was checked IRQ latency difference using TBM in a setup similar to
>>        [5]. Please note that the IRQ rate is one in 30 seconds, and only
>>        VCPU->idle->VCPU use-case is considered. With following results (in ns,
>>        the timer granularity 120ns):
> 
> How long did you run the benchmark?

I did run it until avg more ore less stabilizes (2-3 minutes), then took the minimal avg (note, we have a moving average there).

> 
>>
>>        mapped on access:
>>            max=9960 warm_max=8640 min=7200 avg=7626
>>        mapped on init:
>>            max=9480 warm_max=8400 min=7080 avg=7341
>>
>>        Unfortunately there are no consitent results yet from profiling using
>>        Lauterbach PowerTrace. Still in communication with the tracer vendor in
>>        order to setup the proper configuration.

-- 
Sincerely,
Andrii Anisov.

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