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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=9veu/89fdLOXOwROQXRtrg7V3t8PiokUD1pb3Hncd5Q=; b=nDXb/DxM6FNe+lOUGI9Rvos8qIvZjYz5JQAfkCusRLvbJ4cCyC9LNZJI5ZSJ60VfYz diTGDb0ulp912Jcl7uGKm1gKppmZOISD3h2nxxBTyRpCABTAJIebuJqO5t/rJ+DDHGYO YrldWufHtWgfckuQrNRD5fQqAQztK2+EApbrDK3D9sU0zi3cXfrWJskmA4GJqgZze+F/ crcpFhf7Y5uCpCMrogqQxr1en/IGtr9SqxksTgJp+fyk7SmImI7cbySwcmSFwc0lzzsr t04ENNdw45PpwXPz9mHI6tVNvP1f1K0GYOJNb5yUbF1RdP36J3AGJQg/Y1lpjPNQupHJ bSMQ== X-Gm-Message-State: AOAM530ycM6X6tHU98RRtMkmxOnczTF19D+tDiXaGxKZt4fXMGTU2vnU +E2ig8rVbvsZL0C5W2rdm5HGoJsO88c1oQ== X-Google-Smtp-Source: ABdhPJyFSwyEiwnn3G6isxehumFtenn+UsTTokHJUVstFVwcJs3deSHagC6X0x1sA5mSYceuuP9QMg== X-Received: by 2002:ac2:5462:: with SMTP id e2mr3225826lfn.552.1607619465716; Thu, 10 Dec 2020 08:57:45 -0800 (PST) Subject: Re: [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server() To: paul@xen.org Cc: 'Jan Beulich' , 'Oleksandr Tyshchenko' , 'Stefano Stabellini' , 'Julien Grall' , 'Volodymyr Babchuk' , 'Andrew Cooper' , 'George Dunlap' , 'Ian Jackson' , 'Wei Liu' , 'Julien Grall' , xen-devel@lists.xenproject.org References: <1606732298-22107-1-git-send-email-olekstysh@gmail.com> <1606732298-22107-18-git-send-email-olekstysh@gmail.com> <3bb4c3b5-a46a-ba31-292f-5c6ba49fa9be@suse.com> <6026b7f3-ae6e-f98f-be65-27d7f729a37f@gmail.com> <18bfd9b1-3e6a-8119-efd0-c82ad7ae681d@gmail.com> <0d6c01d6cd9a$666326c0$33297440$@xen.org> <57bfc007-e400-6777-0075-827daa8acf0e@gmail.com> <0d7201d6ce09$e13dce80$a3b96b80$@xen.org> <96b9b843-f4fe-834a-f17b-d75198aa0dab@gmail.com> <002401d6cecf$ce472710$6ad57530$@xen.org> From: Oleksandr Message-ID: <1c012758-7121-5702-038e-4510f3cf1b0b@gmail.com> Date: Thu, 10 Dec 2020 18:57:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <002401d6cecf$ce472710$6ad57530$@xen.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US On 10.12.20 10:38, Paul Durrant wrote: Hi Paul. >> -----Original Message----- >> From: Oleksandr >> Sent: 09 December 2020 20:36 >> To: paul@xen.org >> Cc: 'Jan Beulich' ; 'Oleksandr Tyshchenko' ; >> 'Stefano Stabellini' ; 'Julien Grall' ; 'Volodymyr Babchuk' >> ; 'Andrew Cooper' ; 'George Dunlap' >> ; 'Ian Jackson' ; 'Wei Liu' ; 'Julien Grall' >> ; xen-devel@lists.xenproject.org >> 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