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

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Date:	Thu, 05 Feb 2015 01:57:11 +0100
From:	Alexander Graf <agraf@...e.de>
To:	David Gibson <david@...son.dropbear.id.au>, paulus@...ba.org
CC:	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 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.


Alex
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