linux-kernel - Re: [RFC PATCH 05/13] iommufd: Serialise persisted iommufds and ioas

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Message-ID: <20241007150138.GM2456194@ziepe.ca>
Date: Mon, 7 Oct 2024 12:01:38 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Gowans, James" <jgowans@...zon.com>
Cc: "dwmw2@...radead.org" <dwmw2@...radead.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"rppt@...nel.org" <rppt@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
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	"Graf (AWS), Alexander" <graf@...zon.de>,
	"will@...nel.org" <will@...nel.org>,
	"joro@...tes.org" <joro@...tes.org>
Subject: Re: [RFC PATCH 05/13] iommufd: Serialise persisted iommufds and ioas

On Mon, Oct 07, 2024 at 08:57:07AM +0000, Gowans, James wrote:
> With the ARM SMMUv3 for example I think there are break-before-make
> requirement, so is it possible to do an atomic switch of the SMMUv3 page
> table PGD in a hitless way? 

The BBM rules are only about cached translations. If all your IOPTEs
result in the same translation *size* then you are safe. You can
change the radix memory storing the IOPTEs freely, AFAIK.

BBM level 2 capable HW doesn't have those limitations either and
everything is possible.

Jason