linux-kernel - Re: [RFC PATCH V3 2/2] net: netvsc: Allocate per-device swiotlb bounce buffer for netvsc

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Message-ID: <56243d2b-d7f6-9e45-d5fd-3af7767d52e3@gmail.com>
Date:   Fri, 27 May 2022 20:20:50 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "hch@....de" <hch@....de>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "andi.kleen@...el.com" <andi.kleen@...el.com>,
        "kirill.shutemov" <kirill.shutemov@...el.com>
Subject: Re: [RFC PATCH V3 2/2] net: netvsc: Allocate per-device swiotlb
 bounce buffer for netvsc



On 5/27/2022 2:43 AM, Dexuan Cui wrote:
>> From: Tianyu Lan <ltykernel@...il.com>
>> Sent: Thursday, May 26, 2022 5:01 AM
>> ...
>> @@ -119,6 +124,10 @@ static void netvsc_subchan_work(struct work_struct
>> *w)
>>   			nvdev->max_chn = 1;
>>   			nvdev->num_chn = 1;
>>   		}
>> +
>> +		/* Allocate boucne buffer.*/
>> +		swiotlb_device_allocate(&hdev->device, nvdev->num_chn,
>> +				10 * IO_TLB_BLOCK_UNIT);
>>   	}
> 
> Looks like swiotlb_device_allocate() is not called if the netvsc device
> has only 1 primary channel and no sub-schannel, e.g. in the case of
> single-vCPU VM?

When there is only sinlge，there seems not to be much performance
penalty. But you are right, we should keep the same behavior when single 
CPU and multi CPU. Will update in the next version.

Thanks.