linux-kernel - Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain

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Date:   Thu, 27 Oct 2022 14:44:49 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Gerd Bayer <gbayer@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list
 iteration

On Mon, 2022-10-24 at 13:26 -0300, Jason Gunthorpe wrote:
> On Mon, Oct 24, 2022 at 05:22:24PM +0200, Niklas Schnelle wrote:
> 
> > Thanks for the explanation, still would like to grok this a bit more if
> > you don't mind. If I do read things correctly synchronize_rcu() should
> > run in the conext of the VFIO ioctl in this case and shouldn't block
> > anything else in the kernel, correct? At least that's how I understand
> > the synchronize_rcu() comments and the fact that e.g.
> > net/vmw_vsock/virtio_transport.c:virtio_vsock_remove() also does a
> > synchronize_rcu() and can be triggered from user-space too.
> 
> Yes, but I wouldn't look in the kernel to understand if things are OK
>  
> > So we're
> > more worried about user-space getting slowed down rather than a Denial-
> > of-Service against other kernel tasks.
> 
> Yes, functionally it is OK, but for something like vfio with vIOMMU
> you could be looking at several domains that have to be detached
> sequentially and with grace periods > 1s you can reach multiple
> seconds to complete something like a close() system call. Generally it
> should be weighed carefully
> 
> Jason

Thanks for the detailed explanation. Then let's not put a
synchronize_rcu() in detach, as I said as long as the I/O translation
tables are there an IOTLB flush after zpci_unregister_ioat() should
result in an ignorable error. That said, I think if we don't have the
synchronize_rcu() in detach we need it in s390_domain_free() before
freeing the I/O translation tables.