[RFC 5/7] scsi: storvsc: Enable swiotlb throttling

[Michael Kelley]

From: Petr Tesařík <petr at tesarici.cz> Sent: Friday, August 23, 2024 1:20 AM
> 
> On Thu, 22 Aug 2024 11:37:16 -0700
> mhkelley58 at gmail.com wrote:
> 
> > From: Michael Kelley <mhklinux at outlook.com>
> >
> > In a CoCo VM, all DMA-based I/O must use swiotlb bounce buffers
> > because DMA cannot be done to private (encrypted) portions of VM
> > memory. The bounce buffer memory is marked shared (decrypted) at
> > boot time, so I/O is done to/from the bounce buffer memory and then
> > copied by the CPU to/from the final target memory (i.e, "bounced").
> > Storage devices can be large consumers of bounce buffer memory because it
> > is possible to have large numbers of I/Os in flight across multiple
> > devices. Bounce buffer memory must be pre-allocated at boot time, and
> > it is difficult to know how much memory to allocate to handle peak
> > storage I/O loads. Consequently, bounce buffer memory is typically
> > over-provisioned, which wastes memory, and may still not avoid a peak
> > that exhausts bounce buffer memory and cause storage I/O errors.
> >
> > To solve this problem for Coco VMs running on Hyper-V, update the
> > storvsc driver to permit bounce buffer throttling. First, use
> > scsi_dma_map_attrs() instead of scsi_dma_map(). Then gate the
> > throttling behavior on a DMA layer check indicating that throttling is
> > useful, so that no change occurs in a non-CoCo VM. If throttling is
> > useful, pass the DMA_ATTR_MAY_BLOCK attribute, and set the block queue
> > flag indicating that the I/O request submission path may sleep, which
> > could happen when throttling. With these options in place, DMA map
> > requests are pended when necessary to reduce the likelihood of usage
> > peaks caused by storvsc that could exhaust bounce buffer memory and
> > generate errors.
> >
> > Signed-off-by: Michael Kelley <mhklinux at outlook.com>
> 
> LGTM, but I'm not familiar with this driver or the SCSI layer. In
> particular, I don't know if it's OK to change the value of
> host->queuecommand_may_block after scsi_host_alloc() initialized it
> from a scsi host template, although it seems to be fine.
> 
> Petr T

Yes, it's OK to change the value after scsi_host_alloc().
The flag isn't consumed until scsi_add_host() is called
later in storvsc_probe().

Note this maps to BLK_MQ_F_BLOCKING, which you can see in
/sys/kernel/debug/block/<device>/hctx0/flags. Same for NVMe
devices with my Patches 6 and 7. When debugging, I've been
checking that /sys entry to make sure the behavior is as expected. :-)

Michael

> 
> > ---
> >  drivers/scsi/storvsc_drv.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> > index 7ceb982040a5..7bedd5502d07 100644
> > --- a/drivers/scsi/storvsc_drv.c
> > +++ b/drivers/scsi/storvsc_drv.c
> > @@ -457,6 +457,7 @@ struct hv_host_device {
> >  	struct workqueue_struct *handle_error_wq;
> >  	struct work_struct host_scan_work;
> >  	struct Scsi_Host *host;
> > +	unsigned long dma_attrs;
> >  };
> >
> >  struct storvsc_scan_work {
> > @@ -1810,7 +1811,7 @@ 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);
> > +		sg_count = scsi_dma_map_attrs(scmnd, host_dev->dma_attrs);
> >  		if (sg_count < 0) {
> >  			ret = SCSI_MLQUEUE_DEVICE_BUSY;
> >  			goto err_free_payload;
> > @@ -2030,6 +2031,12 @@ static int storvsc_probe(struct hv_device *device,
> >  	 *    have an offset that is a multiple of HV_HYP_PAGE_SIZE.
> >  	 */
> >  	host->sg_tablesize = (max_xfer_bytes >> HV_HYP_PAGE_SHIFT) + 1;
> > +
> > +	if (dma_recommend_may_block(&device->device)) {
> > +		host->queuecommand_may_block = true;
> > +		host_dev->dma_attrs = DMA_ATTR_MAY_BLOCK;
> > +	}
> > +
> >  	/*
> >  	 * For non-IDE disks, the host supports multiple channels.
> >  	 * Set the number of HW queues we are supporting.