linux-kernel - Re: [PATCHv2] kvmppc: Implement H_LOGICAL_CI

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Message-ID: <20150205004846.GE25675@voom.fritz.box>
Date:	Thu, 5 Feb 2015 11:48:46 +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: [PATCHv2] kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM

On Wed, Feb 04, 2015 at 04:24:46PM +0100, Alexander Graf wrote:
> 
> 
> On 03.02.15 06:44, 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>
> > ---
> >  arch/powerpc/include/asm/kvm_book3s.h |  3 ++
> >  arch/powerpc/kvm/book3s.c             | 76 +++++++++++++++++++++++++++++++++++
> >  arch/powerpc/kvm/book3s_hv.c          | 12 ++++++
> >  arch/powerpc/kvm/book3s_pr_papr.c     | 28 +++++++++++++
> >  4 files changed, 119 insertions(+)
> > 
> > v2:
> >   - Removed some debugging printk()s that were accidentally left in
> >   - Fix endianness; like all PAPR hypercalls, these should always act
> >     big-endian, even if the guest is little-endian (in practice this
> >     makes no difference, since the only user is SLOF, which is always
> >     big-endian)
> > 
> > diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> > index 942c7b1..578e550 100644
> > --- a/arch/powerpc/include/asm/kvm_book3s.h
> > +++ b/arch/powerpc/include/asm/kvm_book3s.h
> > @@ -292,6 +292,9 @@ static inline bool kvmppc_supports_magic_page(struct kvm_vcpu *vcpu)
> >  	return !is_kvmppc_hv_enabled(vcpu->kvm);
> >  }
> >  
> > +extern int kvmppc_h_logical_ci_load(struct kvm_vcpu *vcpu);
> > +extern int kvmppc_h_logical_ci_store(struct kvm_vcpu *vcpu);
> > +
> >  /* Magic register values loaded into r3 and r4 before the 'sc' assembly
> >   * instruction for the OSI hypercalls */
> >  #define OSI_SC_MAGIC_R3			0x113724FA
> > diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> > index 888bf46..7b51492 100644
> > --- a/arch/powerpc/kvm/book3s.c
> > +++ b/arch/powerpc/kvm/book3s.c
> > @@ -820,6 +820,82 @@ void kvmppc_core_destroy_vm(struct kvm *kvm)
> >  #endif
> >  }
> >  
> > +int kvmppc_h_logical_ci_load(struct kvm_vcpu *vcpu)
> > +{
> > +	unsigned long size = kvmppc_get_gpr(vcpu, 4);
> > +	unsigned long addr = kvmppc_get_gpr(vcpu, 5);
> > +	u64 buf;
> > +	int ret;
> > +
> > +	if (!is_power_of_2(size) || (size > sizeof(buf)))
> > +		return H_TOO_HARD;
> > +
> > +	ret = kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, size, &buf);
> > +	if (ret != 0)
> > +		return H_TOO_HARD;
> > +
> > +	switch (size) {
> > +	case 1:
> > +		kvmppc_set_gpr(vcpu, 4, *(u8 *)&buf);
> > +		break;
> > +
> > +	case 2:
> > +		kvmppc_set_gpr(vcpu, 4, be16_to_cpu(*(u16 *)&buf));
> > +		break;
> > +
> > +	case 4:
> > +		kvmppc_set_gpr(vcpu, 4, be32_to_cpu(*(u32 *)&buf));
> > +		break;
> > +
> > +	case 8:
> > +		kvmppc_set_gpr(vcpu, 4, be64_to_cpu(*(u64 *)&buf));
> 
> Shouldn't these casts be __be types?

Ah, yes they should.

> > +		break;
> > +
> > +	default:
> > +		BUG();
> > +	}
> > +
> > +	return H_SUCCESS;
> > +}
> > +EXPORT_SYMBOL_GPL(kvmppc_h_logical_ci_load); /* For use by the kvm-pr module */
> 
> No need for the comment.

Ok.

-- 
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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