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

[Tianyu Lan]

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.