linux-kernel - Re: [PATCHv3] kvmppc: Implement H_LOGICAL_CI

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Message-ID: <20150317014113.GL5741@voom.redhat.com>
Date:	Tue, 17 Mar 2015 12:41:13 +1100
From:	David Gibson <david@...son.dropbear.id.au>
To:	Alexander Graf <agraf@...e.de>
Cc:	paulus@...ba.org, aik@...abs.ru, kvm-ppc@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3] kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM

On Thu, Feb 05, 2015 at 01:57:11AM +0100, Alexander Graf wrote:
> 
> 
> On 05.02.15 01:53, David Gibson wrote:
> > On POWER, storage caching is usually configured via the MMU - attributes
> > such as cache-inhibited are stored in the TLB and the hashed page table.
> > 
> > This makes correctly performing cache inhibited IO accesses awkward when
> > the MMU is turned off (real mode).  Some CPU models provide special
> > registers to control the cache attributes of real mode load and stores but
> > this is not at all consistent.  This is a problem in particular for SLOF,
> > the firmware used on KVM guests, which runs entirely in real mode, but
> > which needs to do IO to load the kernel.
> > 
> > To simplify this qemu implements two special hypercalls, H_LOGICAL_CI_LOAD
> > and H_LOGICAL_CI_STORE which simulate a cache-inhibited load or store to
> > a logical address (aka guest physical address).  SLOF uses these for IO.
> > 
> > However, because these are implemented within qemu, not the host kernel,
> > these bypass any IO devices emulated within KVM itself.  The simplest way
> > to see this problem is to attempt to boot a KVM guest from a virtio-blk
> > device with iothread / dataplane enabled.  The iothread code relies on an
> > in kernel implementation of the virtio queue notification, which is not
> > triggered by the IO hcalls, and so the guest will stall in SLOF unable to
> > load the guest OS.
> > 
> > This patch addresses this by providing in-kernel implementations of the
> > 2 hypercalls, which correctly scan the KVM IO bus.  Any access to an
> > address not handled by the KVM IO bus will cause a VM exit, hitting the
> > qemu implementation as before.
> > 
> > Note that a userspace change is also required, in order to enable these
> > new hcall implementations with KVM_CAP_PPC_ENABLE_HCALL.
> > 
> > Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> 
> Thanks, applied to kvm-ppc-queue.

Any news on when this might go up to mainline?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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