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Date: Wed, 09 Dec 2020 13:25:18 +0000 From: Marc Zyngier <maz@...nel.org> To: Catalin Marinas <catalin.marinas@....com> Cc: Steven Price <steven.price@....com>, Peter Maydell <peter.maydell@...aro.org>, Haibo Xu <haibo.xu@...aro.org>, lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>, Juan Quintela <quintela@...hat.com>, Richard Henderson <richard.henderson@...aro.org>, QEMU Developers <qemu-devel@...gnu.org>, "Dr. David Alan Gilbert" <dgilbert@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>, kvmarm <kvmarm@...ts.cs.columbia.edu>, arm-mail-list <linux-arm-kernel@...ts.infradead.org>, Dave Martin <Dave.Martin@....com> Subject: Re: [PATCH v5 0/2] MTE support for KVM guest On 2020-12-09 12:44, Catalin Marinas wrote: > On Tue, Dec 08, 2020 at 06:21:12PM +0000, Marc Zyngier wrote: >> On 2020-12-08 17:21, Catalin Marinas wrote: >> > On Mon, Dec 07, 2020 at 07:03:13PM +0000, Marc Zyngier wrote: >> > > I wonder whether we will have to have something kernel side to >> > > dump/reload tags in a way that matches the patterns used by live >> > > migration. >> > >> > We have something related - ptrace dumps/resores the tags. Can the same >> > concept be expanded to a KVM ioctl? >> >> Yes, although I wonder whether we should integrate this deeply into >> the dirty-log mechanism: it would be really interesting to dump the >> tags at the point where the page is flagged as clean from a dirty-log >> point of view. As the page is dirtied, discard the saved tags. > > From the VMM perspective, the tags can be treated just like additional > (meta)data in a page. We'd only need the tags when copying over. It can > race with the VM dirtying the page (writing tags would dirty it) but I > don't think the current migration code cares about this. If dirtied, it > copies it again. > > The only downside I see is an extra syscall per page both on the origin > VMM and the destination one to dump/restore the tags. Is this a > performance issue? I'm not sure. Migrating VMs already has a massive overhead, so an extra syscall per page isn't terrifying. But that's the point where I admit not knowing enough about what the VMM expects, nor whether that matches what happens on other architectures that deal with per-page metadata. Would this syscall operate on the guest address space? Or on the VMM's own mapping? M. -- Jazz is not dead. It just smells funny...
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