linux-kernel - Re: [PATCHv4] kvmppc: Implement H_LOGICAL_CI

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Date:	Tue, 21 Apr 2015 16:51:21 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Thomas Huth <thuth@...hat.com>
Cc:	agraf@...e.de, michael@...erman.id.au, benh@...nel.crashing.org,
	aik@...abs.ru, kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4] kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM

On Tue, Apr 21, 2015 at 08:37:02AM +0200, Thomas Huth wrote:
> Am Tue, 21 Apr 2015 10:41:51 +1000
> schrieb David Gibson <david@...son.dropbear.id.au>:
> 
> > 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(+)
> ...
> > diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> > index cfbcdc6..453a8a4 100644
> > --- a/arch/powerpc/kvm/book3s.c
> > +++ b/arch/powerpc/kvm/book3s.c
> > @@ -821,6 +821,82 @@ 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_MMIO_BUS, addr, size, &buf);
> > +	if (ret != 0)
> > +		return H_TOO_HARD;
> > +
> > +	switch (size) {
> > +	case 1:
> > +		kvmppc_set_gpr(vcpu, 4, *(u8 *)&buf);
> > +		break;
> > +
> 
> Most of the code in book3s.c seems not to use a empty line after a
> "break;", so may I suggest to remove these empty lines here, too, to
> keep the coding style a little bit more consistent?

I don't think it's worth respinning just for that.

> > +	case 2:
> > +		kvmppc_set_gpr(vcpu, 4, be16_to_cpu(*(__be16 *)&buf));
> > +		break;
> > +
> > +	case 4:
> > +		kvmppc_set_gpr(vcpu, 4, be32_to_cpu(*(__be32 *)&buf));
> > +		break;
> > +
> > +	case 8:
> > +		kvmppc_set_gpr(vcpu, 4, be64_to_cpu(*(__be64 *)&buf));
> > +		break;
> > +
> > +	default:
> > +		BUG();
> 
> If I got the code right, a malicious guest could easily trigger this
> BUG() statement, couldn't it? ... so a BUG() is maybe not the right
> thing to do here. Would it be appropriate to return an error value to
> the guest instead?

Actually no - the test at the top of the function for
is_power_of_2(size) etc. catches this safely before we get here.  The
BUG() is just paranoia.


> 
> > +	}
> > +
> > +	return H_SUCCESS;
> > +}
> > +EXPORT_SYMBOL_GPL(kvmppc_h_logical_ci_load);
> 
>  Thomas
> 

-- 
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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