netdev - Re: [PATCH V6 4/5] scsi: storvsc: Add Isolation VM support for storvsc driver

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Date:   Thu, 9 Dec 2021 19:17:08 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Long Li <longli@...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>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "hch@...radead.org" <hch@...radead.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "hch@....de" <hch@....de>, "joro@...tes.org" <joro@...tes.org>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>
Subject: Re: [PATCH V6 4/5] scsi: storvsc: Add Isolation VM support for
 storvsc driver



On 12/9/2021 4:00 PM, Long Li wrote:
>> @@ -1848,21 +1851,22 @@ static int storvsc_queuecommand(struct Scsi_Host
>> *host, struct scsi_cmnd *scmnd)
>>   		payload->range.len = length;
>>   		payload->range.offset = offset_in_hvpg;
>>
>> +		sg_count = scsi_dma_map(scmnd);
>> +		if (sg_count < 0)
>> +			return SCSI_MLQUEUE_DEVICE_BUSY;
> Hi Tianyu,
> 
> This patch (and this patch series) unconditionally adds code for dealing with DMA addresses for all VMs, including non-isolation VMs.
> 
> Does this add performance penalty for VMs that don't require isolation?
> 

Hi Long:
	scsi_dma_map() in the traditional VM just save sg->offset to
sg->dma_address and no data copy because swiotlb bounce buffer code
doesn't work. The data copy only takes place in the Isolation VM and
swiotlb_force is set. So there is no additional overhead in the 
traditional VM.

Thanks.