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Date: Tue, 31 Mar 2026 16:29:42 +0300
From: Leon Romanovsky <leon@kernel.org>
To: Keith Busch <kbusch@kernel.org>
Cc: Zhiping Zhang <zhipingz@meta.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	Bjorn Helgaas <bhelgaas@google.com>, linux-rdma@vger.kernel.org,
	linux-pci@vger.kernel.org, netdev@vger.kernel.org,
	dri-devel@lists.freedesktop.org, Yochai Cohen <yochai@nvidia.com>,
	Yishai Hadas <yishaih@nvidia.com>,
	Bjorn Helgaas <helgaas@kernel.org>
Subject: Re: [RFC v2 1/2] vfio: add callback to get tph info for dmabuf
Message-ID: <20260331132942.GC814676@unreal>
References: <20260324234615.3731237-1-zhipingz@meta.com>
 <20260324234615.3731237-2-zhipingz@meta.com>
 <20260325082534.GN814676@unreal>
 <acW2BwQKaUbS3eL9@kbusch-mbp>
 <20260331083758.GA814676@unreal>
 <acvFV8c5QVxnt3Em@kbusch-mbp>
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In-Reply-To: <acvFV8c5QVxnt3Em@kbusch-mbp>

On Tue, Mar 31, 2026 at 07:00:07AM -0600, Keith Busch wrote:
> On Tue, Mar 31, 2026 at 11:37:58AM +0300, Leon Romanovsky wrote:
> > On Thu, Mar 26, 2026 at 04:41:11PM -0600, Keith Busch wrote:
> > > 
> > > You're suggesting that Ziping append the new fields to the end of this
> > > struct? I don't think we can modify the layout of a uapi.
> > 
> > He needs to add before flex array. This struct is submitted by the user
> > and kernel can easily calculate the position of that array.
> 
> No, you can't just do that. Existing applications would break when they
> compile against the updated kernel header. They don't know about this
> new "tph" supplied flag, but they'll all accidently use the new
> dma_ranges offset. 

So we need to always pass TPH flag and treat 0 as do-nothing-field.

Thanks