netdev - Re: [PATCH V4 vfio 05/10] vfio: Introduce the DMA logging feature support

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Message-ID: <20220825164651.384bf099.alex.williamson@redhat.com>
Date:   Thu, 25 Aug 2022 16:46:51 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Yishai Hadas <yishaih@...dia.com>, jgg@...dia.com,
        saeedm@...dia.com, kvm@...r.kernel.org, netdev@...r.kernel.org,
        kuba@...nel.org, kevin.tian@...el.com, leonro@...dia.com,
        maorg@...dia.com, cohuck@...hat.com
Subject: Re: [PATCH V4 vfio 05/10] vfio: Introduce the DMA logging feature
 support

On Thu, 25 Aug 2022 23:26:04 +0100
Joao Martins <joao.m.martins@...cle.com> wrote:

> On 8/25/22 21:49, Alex Williamson wrote:
> > On Mon, 15 Aug 2022 18:11:04 +0300
> > Yishai Hadas <yishaih@...dia.com> wrote:  
> >> +static int
> >> +vfio_ioctl_device_feature_logging_report(struct vfio_device *device,
> >> +					 u32 flags, void __user *arg,
> >> +					 size_t argsz)
> >> +{
> >> +	size_t minsz =
> >> +		offsetofend(struct vfio_device_feature_dma_logging_report,
> >> +			    bitmap);
> >> +	struct vfio_device_feature_dma_logging_report report;
> >> +	struct iova_bitmap_iter iter;
> >> +	int ret;
> >> +
> >> +	if (!device->log_ops)
> >> +		return -ENOTTY;
> >> +
> >> +	ret = vfio_check_feature(flags, argsz,
> >> +				 VFIO_DEVICE_FEATURE_GET,
> >> +				 sizeof(report));
> >> +	if (ret != 1)
> >> +		return ret;
> >> +
> >> +	if (copy_from_user(&report, arg, minsz))
> >> +		return -EFAULT;
> >> +
> >> +	if (report.page_size < PAGE_SIZE || !is_power_of_2(report.page_size))  
> > 
> > Why is PAGE_SIZE a factor here?  I'm under the impression that
> > iova_bitmap is intended to handle arbitrary page sizes.  Thanks,  
> 
> Arbritary page sizes ... which are powers of 2. We use page shift in iova bitmap.
> While it's not hard to lose this restriction (trading a shift over a slower mul)
> ... I am not sure it is worth supporting said use considering that there aren't
> non-powers of 2 page sizes right now?
> 
> The PAGE_SIZE restriction might be that it's supposed to be the lowest possible page_size.

Sorry, I was unclear.  Size relative to PAGE_SIZE was my only question,
not that we shouldn't require power of 2 sizes.  We're adding device
level dirty tracking, where the device page size granularity might be
4K on a host with a CPU 64K page size.  Maybe there's a use case for
that.  Given the flexibility claimed by the iova_bitmap support,
requiring reported page size less than system PAGE_SIZE seems
unjustified.  Thanks,

Alex

> >> +		return -EINVAL;
> >> +
> >> +	ret = iova_bitmap_iter_init(&iter, report.iova, report.length,
> >> +				    report.page_size,
> >> +				    u64_to_user_ptr(report.bitmap));
> >> +	if (ret)
> >> +		return ret;
> >> +
> >> +	for (; !iova_bitmap_iter_done(&iter) && !ret;
> >> +	     ret = iova_bitmap_iter_advance(&iter)) {
> >> +		ret = device->log_ops->log_read_and_clear(device,
> >> +			iova_bitmap_iova(&iter),
> >> +			iova_bitmap_length(&iter), &iter.dirty);
> >> +		if (ret)
> >> +			break;
> >> +	}
> >> +
> >> +	iova_bitmap_iter_free(&iter);
> >> +	return ret;
> >> +}  
> >   
>