linux-kernel - Re: [PATCH 3/3] PPC: KVM: Add support for 64bit TCE windows

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Message-ID: <1401971411.3247.132.camel@pasglop>
Date:	Thu, 05 Jun 2014 22:30:11 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alexander Graf <agraf@...e.de>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	linuxppc-dev@...ts.ozlabs.org, Paul Mackerras <paulus@...ba.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org
Subject: Re: [PATCH 3/3] PPC: KVM: Add support for 64bit TCE windows

On Thu, 2014-06-05 at 13:56 +0200, Alexander Graf wrote:
> What if we ask user space to give us a pointer to user space allocated 
> memory along with the TCE registration? We would still ask user space to 
> only use the returned fd for TCE modifications, but would have some 
> nicely swappable memory we can store the TCE entries in.

That isn't going to work terribly well for VFIO :-) But yes, for
emulated devices, we could improve things a bit, including for
the 32-bit TCE tables.

For emulated, the real mode path could walk the page tables and fallback
to virtual mode & get_user if the page isn't present, thus operating
directly on qemu memory TCE tables instead of the current pinned stuff.

However that has a cost in performance, but since that's really only
used for emulated devices and PAPR VIOs, it might not be a huge issue.

But for VFIO we don't have much choice, we need to create something the
HW can access.

> In fact, the code as is today can allocate an arbitrary amount of pinned 
> kernel memory from within user space without any checks.

Right. We should at least account it in the locked limit.

Cheers,
Ben.

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