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SubjectRe: [PATCHv4] kvmppc: Implement H_LOGICAL_CI_{LOAD,STORE} in KVM
On 04/21/2015 02:41 AM, 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 <>
> ---
> 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(+)
> Changes in v4:
> * Rebase onto 4.0+, correct for changed signature of kvm_io_bus_{read,write}
> Alex, I saw from some build system notifications that you seemed to
> hit some troubles compiling the last version of this patch. This
> should fix it - hope it's not too late to get into 4.1.

Oh, I already fixed it up in my tree, no worries.


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