Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <3c2249fc7abf481b15d4988c2bd6456c48154c44.camel@linux.ibm.com>
Subject: Re: [PATCH 3/5] iommu/s390: Use RCU to allow concurrent domain_list
 iteration
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Matthew Rosato <mjrosato@linux.ibm.com>, iommu@lists.linux.dev,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Gerd Bayer <gbayer@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        svens@linux.ibm.com, linux-kernel@vger.kernel.org
Date:   Thu, 27 Oct 2022 14:44:49 +0200
In-Reply-To: <Y1a8qM4c2ZAM9glJ@nvidia.com>
References: <Y07Dz/NROAMI0Hku@nvidia.com>
         <8e268ab5e0dadf86be5fd7ffaa9debb76cea67f3.camel@linux.ibm.com>
         <Y0/lMCQ8oeXJ2HTg@nvidia.com>
         <f3551bb461b3ef3cfc1a0c644093816be1835b3f.camel@linux.ibm.com>
         <Y1ErcEe82yjJI+ET@nvidia.com>
         <68d91d7a5aadbd46dc34470eccd6b86a84c9e47b.camel@linux.ibm.com>
         <Y1KgX8EwH8T+FgWC@nvidia.com>
         <89a748fb5caee8be5d91806aa5dfd131e92d5d82.camel@linux.ibm.com>
         <Y1K1AqVWEyY0/Uqy@nvidia.com>
         <cef734b9f9b33380c1bbff40b56bb67b3de29341.camel@linux.ibm.com>
         <Y1a8qM4c2ZAM9glJ@nvidia.com>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.