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

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Message-ID: <5507CD30.4060501@suse.de>
Date:	Tue, 17 Mar 2015 02:44:00 -0400
From:	Alexander Graf <agraf@...e.de>
To:	David Gibson <david@...son.dropbear.id.au>
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 16.03.15 21:41, David Gibson wrote:
> 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?

I'm aiming for 4.1.


Alex
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